The implementation of reforms of Economic and Monetary Union (EMU) is constrained by a complex interaction between national and EU laws. This paper presents a composite indicator of procedural constraints that national constitutional laws impose on the EMU-induced constitutional amendments. The Constitutional Amendment Index (CAI) is based on the sequence of veto points – parliamentary formations, referenda, heads of states and constitutional review bodies – and procedural requirements that the EMU-induced constitutional amendments need to comply with at each point. The coding of legal requirements combines the analysis of formal procedures in 28 EU member states with expert judgement on the potential use of these requirements in the light of the prevailing legal culture and past experience. While the relevance of these constraints depends on the specific economic and political circumstances, the legal index provides a comparison of procedural requirements across EU member states as well as a potential control variable for quantitative research.