The European Union (EU) Emissions Trading System (ETS) has been established for more than 15 years, but limited attention has been given to how the changing political environment may affect the policy. We address this gap by investigating how the EU enlargement after 2004 affected the ETS and how the effects have been buffered. We develop a framework of institutional resilience to investigate how the established norms and institutional constellation of the EU legislative triumvirate have been instrumental for buffering the effects of the enlargement on ETS policymaking. We find that the existing power structure and functional complementarity of the EU legislative settings have fostered a consensus-building atmosphere in the ETS decision-making to accommodate preference heterogeneity and to absorb the compositional impact after the enlargement. The findings highlight the importance of contextual factors and institutional settings in ETS analysis and suggest a new perspective for assessing dynamic ETS performance.