The law of 20 August 2008 reformed the representativeness of French unions by imposing an obligation for ‘financial transparency’. Building on exploratory research, we address the question of the organizational and political effects of the new regulation, which point to a traditional debate in union democracy studies: how do administrative and representative rationalities combine within trade union organizations? Drawing on interviews with union leaders and finance officers at various levels in three major labour confederations (CGT, CFDT and CGT‐FO), we describe the different ways unionists have received the new accounting requirements and translated them into organizational practices and norms. Going beyond the traditional theses of compatibility and colonization, we make use of the body of work in critical legal and management studies to develop an endogenous approach of the relationships between trade unions and accounting management.