This article examines the Commission's preferences and preference formation in relation to the Convention and the negotiation of the Constitutional Treaty. Opposing rational choice accounts, which explain Commission action in terms of the tendency of bureaucratic actors to seek to maximize power, status and opportunities, it argues that the Commission is best seen as an internally differentiated arena, from which preferences emerge as a result of complex interactions that entail the use of power, institutionalized myths and routines. It contends that the Commission was an ineffective performer in the debate on the future of Europe. As well as committing tactical and strategic mistakes, the Commission was disadvantaged by the explicitly political nature of the exercise and the opportunity structure of the Convention compared to previous IGCs. A third argument is that the ratification and post-ratification process reveal the current limitations on the Commission's ability to influence debates about the future of the Union. Its historic vocation as the engine of integration implies one course of action, while being cast as part of the problem suggests another.