Due diligence before buying carbon credits: what to check

The minute

• Check the registry (Verra, Gold Standard, ACR, CAR) and the project’s status directly on the public database, not just the seller’s material.
• Confirm the serial numbers, the methodology version and whether the credits carry the ICVCM CCP label or a host country authorisation.
• Check the vintage: older credits usually cost less, but some buyers require a recent vintage by internal policy, and CORSIA only accepts reductions from 2021 onward.

Why it matters: Most recent carbon credit controversies started with a buyer who trusted only the seller’s pitch, without checking the primary source at the registry.

Definition: carbon credit due diligence is the set of checks a buyer runs against primary sources (the registry, the project documents and the standard’s own rules) to confirm that a credit exists, has not already been retired, and can support the claim the buyer intends to make. If you need the basics first, start with what carbon credits are and how they work. For the buying routes themselves (marketplaces, brokers, developers), see our guide on how to buy carbon credits. This article covers what to check before you pay.

Where to check: the registry is the primary source

Every serious programme has a public registry showing project status, issuance history and retirements. The ICVCM makes this a Core Carbon Principle: a programme “shall operate or make use of a registry to uniquely identify, record and track mitigation activities”, and information on credited activities must be publicly available. Skipping the registry and trusting only the seller’s PDF is the first common mistake.

On each registry, a project page lists issuances by vintage with a serial number range, and lists retirements separately. If a seller offers credits from a specific project, the serial numbers in the purchase agreement must fall inside an issued block that has not yet been retired. If the numbers do not match, the transaction should stop.

The due diligence checklist, step by step

Step by step for a first purchase

1. Get the project ID and registry name from the seller.
2. Open the registry’s public database and search for that project ID.
3. Confirm the project is registered (not only listed as under development or under validation, and not on hold or inactive).
4. Check the issuance table: verify that the vintage year and quantity the seller offers actually exist as issued credits.
5. Check the retirement records: confirm those specific serial numbers have not already been retired by another buyer.
6. Note the methodology and version, and check whether the ICVCM has approved, rejected or excluded it.
7. If the project has an independent rating, read the full report, not just the letter grade.
8. Download the most recent verification report and note the name of the auditing body (the VVB, Validation and Verification Body). Verra describes a VVB as an independent, third-party auditor approved by Verra.
9. After payment, confirm in the public registry that the retirement was recorded with your organisation as beneficiary.

For a small, one-off purchase this sequence can be done in about an hour. For a large or recurring supply contract, each step deserves deeper review, and engaging a third-party advisor is common practice.

Worked example: checking a seller’s offer against the registry

Illustrative only. The project, figures and serial blocks below are hypothetical and chosen to show the logic, not real data.

A seller offers 5,000 VCUs of vintage 2022 from a forestry project and says they are eligible for CORSIA. The buyer opens the project page in the registry.

The CORSIA row follows ICAO’s CORSIA Eligible Emissions Units document (April 2026), which for the 2024 to 2026 compliance period excludes Verra, ACR and CAR units from 2021 onward that have not been authorised by the host country through an attestation to the avoidance of double-claiming. A vintage 2020 credit would fail this check regardless of authorisation.

Methodology, CCP label and ratings

Methodology version

The methodology decides how the baseline and the reductions were calculated, so its version matters. The ICVCM has shown that methodology choice can decide eligibility for the CCP label. In November 2024 it approved three REDD+ methodologies (ART TREES v2.0, Verra VM0048 and Verra’s JNR Framework v4.1) and stated that credits issued under older Verra REDD+ methodologies (VM0006, VM0007, VM0009, VM0015 and VM0037) will not be able to receive the CCP label. In August 2024 it concluded that eight renewable energy methodologies did not meet its additionality requirements. Our additionality guide explains why this test decides so much.

The CCP label

The ICVCM’s ten Core Carbon Principles cover governance, emissions impact (additionality, permanence, robust quantification, no double counting) and sustainable development. A credit can carry the CCP label only if two conditions hold: the programme is CCP-Eligible and the credit category (methodology) is CCP-Approved. As of 11 May 2026, ICVCM listed ACR, ART TREES, CAR, Equitable Earth, Gold Standard, GCC, Isometric, Puro.Earth, Rainbow and Verra as CCP-Eligible, and estimated that 107 million credits had been approved to use the label. Version also matters at programme level: for Puro.earth, ICVCM stated that only credits issued under CCP-Approved methodologies and version 4.2 or later of its standard will qualify. See also our news on Verra VCS Version 5 and CCP eligibility.

Independent ratings

Independent project ratings try to summarise additionality, permanence and co-benefit risk into a single score, and they influence the price per tonne of each listing. A rating summarises information and analysis; it is not a guarantee. Read the reasoning, the date of the rating and whether it covers the vintage you are buying. Price ranges by quality tier are in our 2026 price guide.

Permanence and buffer

For forest and soil projects, ask how reversal risk is covered. Under Verra’s AFOLU Non-Permanence Risk Tool (v4.2), each project’s risk rating sets its contribution to a pooled buffer account; the minimum rating is 12, and a project with an overall rating above 60 fails the risk analysis. The risk analysis should be among the project documents.

Corresponding adjustments and letters of authorisation

Article 6.2 of the Paris Agreement requires robust accounting to avoid double counting when mitigation outcomes move between countries. The Article 6.2 guidance covers outcomes authorised for use towards NDCs and for “other international mitigation purposes”, such as CORSIA, and requires the participating countries to apply corresponding adjustments. In practice, the host country’s authorisation is documented in a letter of authorisation, the term ICAO uses in its eligibility criteria.

Without a corresponding adjustment, the emission reduction may also count towards the host country’s national target. That does not make the credit fraudulent, but it limits the claim: a buyer can present it as a contribution to climate action in the host country, not as a transferred offset. When a seller says credits are “adjusted” or “Article 6 authorised”, ask for the letter of authorisation and check that it names the project, the volumes, the vintages and the authorised use. Our Article 6 explainer covers the mechanics.

Brazil: what changes in practice

Brazil enacted Law 15,042 on 11 December 2024, creating the SBCE (Sistema Brasileiro de Comércio de Emissões), the country’s regulated carbon market. For due diligence, four provisions matter:

• Registry. The law creates the SBCE Central Registry (Registro Central do SBCE). A voluntary credit is only recognised as a CRVE if it follows a methodology accredited by the SBCE managing body, is verified by an independent entity and is registered in that Central Registry (Art. 44).
• Voluntary use. If SBCE assets are used for voluntary offsetting, they must be cancelled in the Central Registry (Art. 45).
• International transfers. Credits generated in Brazil and used for international transfers must be registered as CRVEs, subject to prior authorisation by the national authority designated for Article 6 (Art. 44, sole paragraph, and Art. 51). The law states that creating, issuing or registering a credit gives no right to that authorisation, which must be formal and specify volumes, deadlines and conditions.
• Timeline. The law sets five implementation phases (Art. 50), starting with up to 12 months, extendable by 12, to issue regulations. Regulation is being published in stages, for example Decree 12,768 of 5 December 2025 on the SBCE Permanent Technical Consultative Committee.

For buyers operating in Brazil, due diligence therefore has an extra layer: besides checking the international registry, track the SBCE rules on methodology accreditation and the Central Registry, because credits that qualify today under voluntary standards may face additional requirements to count within the SBCE. The pillar article on Brazil’s regulated carbon market follows those steps.

Documents to request from the seller

Red flags

• The seller will not share the project ID, the registry or the serial numbers before payment.
• The offered volume or vintage does not appear in the registry’s issuance records.
• Credits are described as “CCP”, “CORSIA-eligible” or “adjusted”, but the registry and documents do not show it.
• The methodology is one the ICVCM has excluded or rejected, and the seller presents the credits as top-tier.
• Verification reports are old, missing or signed by an auditor you cannot identify.
• The seller proposes to keep the credits in its account and send you only a PDF “certificate”.
• The price is far below comparable listings of the same type and vintage, with no explanation.
• The project has faced a methodology correction, suspension or public investigation that the seller does not mention.

What to ask the seller

What baseline was used, who verified it (the auditor’s name), and whether the project has faced any methodology correction, suspension or public investigation. A seller who hesitates to answer is a warning sign. Also ask whether the credits carry a corresponding adjustment, and whether the project has a buffer or insurance mechanism for permanence risk.

The most common mistake

Beyond skipping the registry check, the most frequent error is treating all carbon credits as interchangeable. A credit from an industrial gas destruction project and a credit from a community forestry project carry different risk profiles, co-benefit claims and price ranges, even with the same vintage year. Buyers who select credits by price alone, without reading the project type and methodology, often end up with credits that do not match their stated climate commitments, and face reputational exposure when the mismatch becomes public.

What remains unresolved

Corresponding adjustments are still being implemented country by country. Until host countries confirm what they will authorise for voluntary market transactions, and not only for government-to-government trades or CORSIA, the double counting question has no universal answer. Buyers cannot resolve this on their own; they can document the status at the time of purchase and monitor developments. In Brazil, the SBCE rules on methodology accreditation and the share of CRVEs accepted for compliance are still to be defined by regulation.

Frequently asked questions

How long does basic due diligence take?

For a small purchase, checking the registry, the methodology and the rating can take under an hour; for high volume, a dedicated assessment is worth commissioning.

Do I need a lawyer to buy carbon credits?

Not for small, one-off purchases; for long-term supply contracts, offtake agreements or purchases with a letter of authorisation, legal review is advisable.

Does a high rating guarantee a good project?

It reduces risk but does not eliminate it; a rating summarises information and analysis, it is not a guarantee.

Is a retirement certificate from the seller enough?

No. Match the certificate against the public retirement record in the registry: your organisation as beneficiary, the date and the serial numbers.

Do all credits need a corresponding adjustment?

No. It is required for units used towards another country’s NDC and for uses such as CORSIA. For a voluntary claim, a credit without it can still be used, but the claim should reflect that the host country may also count the reduction.

Read next: where and how to buy carbon credits.

Sources

• ICVCM, Core Carbon Principles: tracking, transparency, CCP label.
• ICVCM, assessment decisions (11 May 2026): CCP-Eligible programmes, credits approved for the label.
• ICVCM, REDD+ decision (15 November 2024)
• ICVCM, renewable energy decision (6 August 2024)
• ICVCM, Puro.earth CCP-Eligible (11 December 2025)
• Verra, Verified Carbon Standard: VVB definition.
• Verra, AFOLU Non-Permanence Risk Tool v4.2: buffer rules.
• Gold Standard, Impact Registry
• ICAO, CORSIA Eligible Emissions Units (April 2026): first-phase unit dates and authorisation.
• UNFCCC, Paris Agreement text
• UNFCCC, Article 6.2 guidance (CMA.3)
• Brazil, Law 15,042/2024: Arts. 44, 45, 50 and 51.
• Brazil, Decree 12,768/2025