Thailand’s sustainability-disclosure calendar produced almost nothing in the last three weeks. The SEC issued no ISSB step, no 56-1 amendment, no hearing. The Federation of Accounting Professions issued no sustainability standard — it published revised financial reporting standards in the Royal Gazette on 26 August, which is a different thing entirely. And on 7 September the Director-General of the Department of Climate Change and Environment, Dr Phirun Saiyasitthipanit, told a Bangkok forum that the Climate Change Act — carbon tax, emissions trading, mandatory reporting — is now expected to come into force in Q3 2027, with the drafting committee still working through chapter 8 of 14.
So if you are a sustainability or investor-relations lead at a Thai listed company, the honest position is this: the next hard, dated event on your disclosure calendar is not a regulation at all. It is a stock index review in December 2026 — and it determines whether your company is in Group 1 of Thailand’s IFRS S1/S2 phase-in.
Most coverage treats that as settled: SET50 equals Group 1, fifty companies, done. Having gone back to the SEC’s own documents and to SET’s own constituent files, we do not think that is right. Here is what the December review actually decides, and what it leaves open.
The next hard date is an index review, not a regulation
SEC press release No. 298/2025, issued 28 November 2025, deferred the first two groups of Thailand’s ISSB roadmap by one year each. The ladder now runs: SET50 reports for fiscal year 2027, filed in 2028; SET100 for FY2028; all SET-listed companies including IPOs for FY2029; mai, REITs, infrastructure funds and property funds for FY2030. Groups 3 and 4 were not moved.
A caution on reading the source. The SEC publishes that ladder only as an image, in Buddhist Era years — the table says 2570, not 2027. It never enters a search index, which is precisely why consultancy blogs across the market still quote the dead FY2026 date from the superseded November 2024 draft. If you see FY2026 for SET50 anywhere, including in your own board papers, it is out of date. We keep the corrected timeline here →
On cohort membership the release is a single sentence: a company is in Group 1 if it is “classified as a constituent of SET 50 in December 2026 Index Review.” That review uses market data through end-November 2026, is announced in mid-December on recent practice, and takes effect 1 January 2027.
Read that sequence again, because it is the operationally awkward part. A company added at the December 2026 review learns it is in Group 1 roughly two weeks before the reporting year it must report on begins. That sits badly with the SEC’s own stated rationale in its earlier consultation — that companies should have “at least 6 months” to prepare and collect data.
Fifty names today. Fifty-nine names over two years.
The SET50 is not a stable list, and this is the point the “fifty companies” framing misses. We pulled SET’s own constituent and inclusion/exclusion files for the last four semi-annual reviews and counted.
Across those four reviews, 14 names entered and 14 left — a mean of 3.5 per review, range two to four. The most recent review, effective 1 July 2026, brought in BCP, MRDIYT, TFG and THAI, and removed BTS, CBG, CENTEL and SAWAD. Widen the lens and the arithmetic is stark: 59 distinct companies have held a SET50 seat at some point since the start of 2025, but only 43 held one continuously. Sixteen moved in or out at least once.
Nor is the list fixed between reviews. On 2 April 2025 SET replaced INTUCH mid-period following its amalgamation with GULF, promoting VGI from the reserve pool. The reserve pool is not a published shortlist — under SET’s Ground Rules it is defined as the securities ranked 51st to 100th by market capitalisation, which is to say the SET100 minus the SET50: fifty companies, about ฿1.49 trillion, some 7.4% of SET market capitalisation.
If you are in the SET100 but outside the SET50, you are one ordinary review away from a mandatory IFRS S1 and S2 reporting obligation — and you will find out about it a fortnight before the reporting year starts.
Othello International — analysis of SET constituent files, September 2026The sentence that is in the draft but not in the press release
Here is the finding that changed how we advise on this. The SEC’s press release is not the most detailed source. The draft-notification consultation — อนร. 37/2568, opened 22 September 2025 — is one step closer to law, and it says something the press release does not.
First, it uses two review rounds as the trigger, not one: a company is caught if it is classified in the December round or the following June round. Second, and more consequentially, it adds a stickiness rule, stating that once a company is classified during the specified periods, the duty to disclose “shall remain even if it is subsequently declassed from the Index.” The SEC recorded that it had decided to maintain that principle after consultation.
Neither sentence appears in press release 298/2025. We cannot resolve from primary sources whether the SEC narrowed to a single review or whether “December 2026” is shorthand for the two-round window shifted by the deferral. The six-month preparation rationale is an argument for the latter. Either way, the commercially important half of the rule — you do not get out by falling out of the index — currently exists only in a consultation paper.
| SEC press release 298/2025 | Draft notification อนร. 37/2568 | |
|---|---|---|
| Date | 28 November 2025 | 22 September 2025 |
| Trigger review | December 2026 only | December round or the following June |
| If you leave the index | Not addressed | Duty remains |
| Status | Announcement | Draft, consultation closed |
| In force? | No | No |
How a company actually lands in the SET50
If membership is the thing that matters, it is worth knowing the tests. SET revised them in its March 2026 Ground Rules, and the thresholds in wide circulation are the superseded 2023 set — trading value of 50% of the market average, turnover of 2%, met in nine of twelve months. Those numbers are wrong now. The current tests are looser on value and volume but measured over a shorter frame.
Only the 200 largest eligible securities enter the review, on a 3-month daily average before the review period.
Not less than 20% of paid-up capital, on the latest data before constituent selection.
Monthly trading value ≥25% of the market average and volume ≥1% of registered shares, in 3 of 4 trading months over 12 months.
At least 100 securities must clear liquidity; the largest 50 of those become the SET50.
Add the basic qualifications — listed at least six months, no delisting process, no prolonged suspension, and no disclaimer or adverse audit opinion — and you have a reasonable self-assessment. A company sitting between roughly 40th and 70th by market capitalisation should treat Group 1 as a live possibility rather than someone else’s problem.
None of this is law yet
One more thing that ought to temper the urgency you are being sold this quarter, including by us. None of this has been gazetted. The SEC’s own release says it “is in the process of amending relevant regulations.” The nine draft notifications and nine draft forms from the September 2025 consultation — including Form 56-1 One Report-S — remain drafts. The IFRS Foundation’s jurisdictional profile for Thailand still reads “in progress.”
So the FY2027 start date, the ladder, the reliefs and the assurance requirement all rest today on a press release and two consultation papers, not on law. They are very likely to hold. They are not yet binding, and anyone telling you otherwise has not checked. We track the status changes here →
What is settled enough to act on: the disclosure will sit inside the 56-1 One Report; limited assurance on Scope 1 and 2 is required from the first year with no relief; and the SEC has explicitly named TGO-registered verifiers as acceptable alongside ISAE 3410, ISSA 5000, ISO 14064-3 and AA1000. TGO CFO certification is the practical route into that →
What a borderline company should do with the next ninety days
A note on language, stated precisely, because loose versions of this claim circulate widely — ours included. The SEC imposes no language condition on ISSB disclosure. Notification TorJor. 44/2556 makes Thai the default for the 56-1 One Report and an English edition explicitly optional. But clause 38 attaches a real duty to the optional English version: its substance must not differ from the Thai, and the company certifies its accuracy.
Separately, FTSE Russell’s ESG Scores FAQ v1.5 lists its key research languages as English, French, Spanish, German, Japanese and Chinese. Thai is not among them — and the earlier 2022 version listed seven, so the list is narrowing, not widening. FTSE does not send a questionnaire; it works from public disclosure only. We are not going to tell you that a theme disclosed in Thai “scores as zero” — FTSE says no such thing anywhere, and the claim does not survive an audit. The defensible point is narrower and, we think, more useful: you do not control which of your two editions an analyst reads, so both had better say the same thing. More on how the English edition is actually used →
- Check your market-cap rank now — the December 2026 measurement window is running today and closes 30 November.
- If you are in the SET100 but outside the SET50, plan as though Group 1 applies. Two to four seats change hands every review.
- Read the draft notification, not just the press release: the stickiness rule means leaving the index may not end the duty.
- Do not budget on FY2026 — SET50 reports for FY2027, filed 2028. Correct any board paper still carrying the old date.
- Line up limited assurance on Scope 1 and 2 early; it applies from year one with no transition relief.
- Reconcile your Thai and English editions clause by clause — clause 38 makes substance parity your legal responsibility.
Find out whether Group 1 applies to you
We map your market-cap rank and liquidity against the current SET50 tests, then audit your Thai and English editions clause by clause so both say the same thing. No questionnaire required — just your published disclosure.
Run the free Gap AuditThe December review will pass with very little noise. SET will publish a list; a few names will change; the market will move on. But for two to four companies it will be the moment a voluntary sustainability chapter becomes a mandatory, assured, standard-referenced disclosure — with about a fortnight’s notice. The work that makes that survivable is the work you do before the list is published, not after. See how the rating frameworks compare → · IFRS S1 & S2 bilingual readiness →
Sources: SEC Thailand News No. 298/2025 (28 Nov 2025) and public hearing อนร. 37/2568 (22 Sep 2025); SET, Ground Rules for SET Index Series (March 2026) and SET50/SET100 constituent and inclusion/exclusion files for H1 2025 – H2 2026; SET market statistics to 31 Aug 2026; ADB Brief No. 387 (April 2026); FTSE Russell ESG Scores and Indices FAQ v1.5 (Dec 2025); SEC Notification TorJor. 44/2556; Department of Climate Change and Environment, remarks of 7 Sep 2026. Constituent churn figures are our own count from SET’s published files. The ISSB notification was not gazetted as at 11 September 2026.
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