This paper discusses the accession of Myanmar to ASEAN and its effects on the emergence of fundamental political norms in ASEAN. The article first advances the theoretical idea of how accession demands towards Regional Organizations (ROs) open windows of opportunity for norm entrepreneurs to act for norm change in a Regional Organization. Secondly, these ideas are empirically illustrated by a case study on ASEAN’s enlargement to include Myanmar. The tentative conclusion is that, being confronted with the accession of the highly stigmatised state of Myanmar, ASEAN was forced to position itself vis-à-vis this outcome, and to open itself up to discussions about fundamental norms and regional identity. This critical phase of transition mobilised norm entrepreneurs to push for norm change, and opened avenues for these entrepreneurs to contribute ideas regarding ASEAN’s regional identity. By subsequently framing the discourse on human rights and democracy in Southeast Asia, and by successfully gaining access to the official ASEAN agenda, these entrepreneurs pressured ASEAN to be more precise about its norms. The discussion about stigmatised Myanmar thus introduced new ideas into the regional forum, and in the longer term may have contributed to ASEAN’s identity-building and the concession of greater attention to democracy and human rights issues.