Reflexive environmental law (REL) techniques create potential for reflexivity by regulated companies. This reflexivity is fundamental to overcoming various regulatory hurdles posed by complex and dynamic sustainability challenges, such as plastics circularity. However, even when European Union (EU) environmental legislation incorporates REL techniques, Member State transposition may either undermine or enhance these techniques' potential for driving regulatee reflexivity. This article examines REL's evolution within complex, transnational legislative frameworks, taking the EU's Single-use Plastics Directive (SUPD) as a case study. It compares reflexivity techniques in the SUPD to those in transpositions by France and Germany. Our analysis is based on the coding of 11 REL techniques and three overarching reflexive drivers—autonomy, accountability and adjustability—within four SUPD instruments. Identifying divergence in REL between the SUPD and Member State transpositions allows us to highlight three explanations as to why REL evolves in certain ways through the transposition process.