Most of the Thai climate-disclosure conversation this year has been about what listed companies will have to report. The more consequential change happened quietly in the machinery underneath it: on 9 April 2026, the Federation of Accounting Professions issued TSSA 5000 — Thailand’s general standard for sustainability assurance engagements — and revoked TSAE 3410, the greenhouse-gas assurance standard that has governed Thai GHG verification since 2024.
TSSA 5000 takes effect for sustainability information reported on or after 15 December 2026, or for periods beginning on or after that date. Read that against the SEC’s phase-in and the implication is sharp: the first mandatory climate disclosure by Thai listed companies will be verified under a standard that is four months away from taking effect, not the one the market has spent two years learning.
Underneath it, the same shift is happening globally. The IAASB has withdrawn ISAE 3410 — the international parent of TSAE 3410 — in favour of ISSA 5000, its first general sustainability assurance standard. Thailand has simply moved in step. Here is what that means for the teams who will have to live with it.
The standard changed in April, and almost nobody noticed
TFAC Notification 13/2569 issued TSSA 5000 and revoked the 2023 notification that had brought in TSAE 3410; a companion notification, 14/2569, carried the conforming amendments. TSAE 3410 covered one thing — a greenhouse-gas statement. TSSA 5000 is deliberately broader: a general standard for assurance over sustainability information, framework-neutral, built to sit over an IFRS S2 climate disclosure as comfortably as over a GHG inventory.
That breadth matters more than it sounds. A GHG-only engagement lets a company treat verification as a laboratory exercise sitting beside the report. A general sustainability assurance standard pulls the assurance boundary around the disclosure — the numbers, the basis of preparation, the organisational boundary, and the narrative that explains them. It is a materially different engagement to scope, and a materially different set of working papers to assemble.
| TSAE 3410 (retiring) | TSSA 5000 (from 15 Dec 2026) | |
|---|---|---|
| What it covers | Greenhouse gas statements | Sustainability information generally, GHG included |
| Issued by TFAC | 8 June 2023 | 9 April 2026 · Notification 13/2569 |
| Effective from | Periods beginning 1 Jan 2024 | 15 December 2026 |
| IAASB parent | ISAE 3410 — withdrawn | ISSA 5000 |
| Bearing on FY2027 | Superseded before the first filing | The standard your first assurance sits under |
One caveat worth stating plainly, because it affects planning: TFAC’s own summary page describes TSAE 3410 as revoked when TSSA 5000 takes effect, while the operative clauses of Notification 13/2569 read as a repeal from the announcement date. If you have an engagement letter in the drafting stage right now, get your assurance provider to confirm in writing which standard the engagement is being performed under. That is a five-minute question in August and an awkward one in 2028.
Verification is the one duty with no transition relief
The Thai SEC’s draft rules are generous on almost everything. Its consultation proposes five transition reliefs — one running a year, four running five years — that let a company ease into ISSB-aligned reporting. Read the list carefully and the omission is the story.
Climate information only under IFRS S2; other sustainability topics stay on comply-or-explain.
Scope 1 and Scope 2 only at the start; Scope 3 follows once the relief lapses.
File separately from the financial statements, and use methods already recognised locally — TGO CFO among them.
Not on the list. It applies in full from a company’s first mandatory reporting year.
The draft Form 56-1 One Report-S is explicit about what is expected: GHG emissions reported under the GHG Protocol, with the verifier named, and the work performed as external limited assurance to an internationally accepted standard. The SEC’s November 2025 statement of revised principles uses the plainer Thai formulation — standardised, reliable verification — but points the same way. Neither is law yet: the consultation closed on 22 October 2025 and, as of today, no final notification has been issued. What has been settled is the direction and the sequencing, and the sequencing is what should be driving your budget.
So: Scope 3 can wait five years. Full sustainability reporting can wait five years. The verifier’s signature cannot wait at all. For a company whose first mandatory year is FY2027, the verification engagement has to be scoped and contracted inside 2027 — which in practice means a provider selected in 2026 and a data system that can survive being audited.
Who is actually allowed to sign your GHG numbers
Thailand’s verifier market is small and it is publicly acknowledged to be small. The TGO register currently lists 22 organisations approved to verify organisational carbon footprints — a mix of international certification bodies and Thai institutes and universities. The SEC clearly saw the constraint coming: rather than restricting assurance to a closed list, its draft permits either a TGO-registered verifier หรือ another verifier working to an accepted international standard, with the accepted list running to ISAE 3410 or ISSA 5000, ISO 14064-3, AA1000, or equivalents the SEC later prescribes. The stated rationale in the consultation is capacity: demand is expected to exceed what the TGO-registered pool can serve.
The exchange agrees. On 3 August 2026, SET and TGO signed a three-year agreement whose explicit aims include developing talent and increasing the number of certified verifiers. When the market operator and the national GHG authority sign a memorandum to manufacture more verifiers, the queue is real. Twenty-two bodies can absorb fifty SET50 companies. They cannot absorb the FY2029 tier, when the duty reaches the whole of the SET.
It is also worth knowing what the market actually looks like today, because the migration lands on real engagement letters. We read the published FY2024 and FY2025 assurance statements of 18 large SET-listed groups — a deliberately chosen sample of the biggest reporters, not a random one. Every single engagement was performed by a certification body rather than an audit firm: LRQA, SGS, TÜV NORD, BSI and Bureau Veritas between them signed all of them. Roughly a third cite ISAE 3000 together with ISAE 3410 — the precise combination now being retired — while most of the rest are AA1000AS report-level engagements at a moderate level. Reasonable assurance appeared twice in eighteen. Whatever else the FY2027 cohort has to solve, a good share of it starts with re-papering an engagement that currently points at a withdrawn standard.
The demand side is not reassuring either. In FY2024, 516 of 915 listed companies reported at least Scope 1 and Scope 2 emissions, and only 332 had those numbers verified by a third party. KPMG’s 2024 survey of Thailand’s largest 100 companies found the share publishing assurance over sustainability disclosures had fallen from 61% to 44% — the steepest decline among the 58 jurisdictions it surveyed. Reported at SET’s November 2025 briefing, GHG reporting already costs a listed company an average of about ฿460,000 and 94 person-days a year, before assurance is priced in. Getting the TGO CFO foundation right early is what keeps that number from compounding.
Fix your calendar: SET50 is FY2027, not FY2026
One correction, because it is everywhere and it is wrong. A great deal of published commentary — including a jurisdictional profile still live on a major standard-setter’s website — states that SET50 companies report first for ปีงบการเงิน 2569. That was the November 2024 consultation. It was superseded.
The SEC’s revised principles, approved in November 2025, anchor the first cohort to the companies classified in the SET50 at the December 2026 index review, with the first mandatory accounting year FY2027 and the first report filed in 2028. SET100 follows for FY2028, the wider SET for FY2029, and mai and the fund vehicles for FY2030. The practical consequence is that the binding cohort is not today’s SET50 list — it is whoever is in the index at that December review, roughly four months away.
That extra year is not a reprieve. It is exactly enough time to build a GHG inventory that can withstand assurance, and not much more. The SET50 timeline in detail และ the wider regulatory tracker set out what lands when.
Assured in Thai, unscored in English
Here is where verification meets the thing we spend our days on. An assured GHG number is an expensive asset. Whether it earns anything depends on where it is legible.
From 2026, FTSE Russell scores SET-listed companies from published disclosure. It sends no questionnaire. Its own methodology is unambiguous about the input:
Data used to calculate FTSE Russell ESG scores is gathered from publicly available sources at the entity level, including corporate reports, websites and press releases.
FTSE Russell · ESG Data Model Methodology v1.2, June 2026Companies do get a review window of two to four weeks, and it is not extendable — but it only lets you point analysts at information you have already published. You cannot close a gap during the window; you can only close it before. And the key research languages FTSE Russell names in its own published FAQ do not include Thai.
So the failure mode is specific and expensive. A company pays for verification, files an assured Scope 1 and 2 figure in a Thai 56-1 One Report, and publishes an English edition where the verifier is unnamed, the assurance level is unstated, the organisational boundary is described loosely, or the emissions table is a summary rather than the disclosure. The assurance was real. The evidence FTSE can score was not. Points are awarded per indicator evidenced, and on the FTSE4Good Thailand Index — launched 6 July 2026 with 119 constituents — entry needs an ESG Score of 2.9 or above, with constituents at risk of deletion below 2.4. The English edition is what earns that scoreและ the path from a TGO carbon number to a scoreable disclosure runs straight through it.
- Confirm with your assurance provider, in writing, whether your next engagement runs under TSAE 3410 or TSSA 5000.
- Budget verification as a first-year cost, not a phased one — it is the only duty with no transition relief.
- Book verifier capacity early: 22 registered bodies, and the exchange itself is trying to grow the pool.
- Reset the calendar to FY2027 for SET50, fixed by the December 2026 index review — and build the inventory now.
- Publish the assurance statement, the verifier’s name and the emissions boundary in อังกฤษ, or the verification earns nothing in the score.
See what your English disclosure is actually scoring
FTSE Russell scores SET-listed companies from public English disclosure — no questionnaire, and a review window that only counts what you have already published. Run the free Gap Audit and find the themes where the work exists but the evidence does not.
ทดลองใช้ Gap Audit ฟรีNone of this is settled law yet, and we would rather say so than sell a deadline that has not been gazetted. But the parts that are settled are the parts with the longest lead times: a new assurance standard effective in December, a verifier market the regulator has already widened because it expects a shortage, and a first cohort fixed by an index review four months away. Those are 2026 decisions, whatever year the filing lands in. Start with IFRS S1 and S2 bilingual disclosure readinessหรือ see how the raters differ →
Sources: TFAC (Notifications 13/2569 and 14/2569; TSSA 5000; TSAE 3410); IAASB (withdrawal of ISAE 3410, ISSA 5000); Thai SEC (public consultation อนร. 37/2568 and draft Form 56-1 One Report-S; news release 298/2568, 28 November 2025); SET (Thailand ISSB Adoption Roadmap; November 2025 GHG briefing as reported by Bangkok Post, The Standard and SD Perspectives; SET–TGO agreement, 3 August 2026); TGO verification-body register; KPMG Survey of Sustainability Reporting 2024; LSEG / FTSE Russell (ESG Data Model Methodology v1.2, June 2026; FTSE4Good Thailand Index Ground Rules v1.0 and factsheet).
See where your English edition is losing FTSE points.
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