The co-decision procedure has had significant implications for the interaction between the EU institutions and has attracted the attention of a series of formal, rational choice institutionalists. However, these have mostly dealt with the Commission in a relatively superficial way and their conclusions about its legislative role have been rather pessimistic. Instead this study examines the role of the Commission in more detail by looking closer at both the formal and informal ways in which the Commission has affected legislation in co-decision from Maastricht to one year after the entering into force of the Amsterdam Treaty. The study includes interview and quantitative data at a general level as well as from three Socrates procedures completed in 1995, 1998, and 2000. In line with the formal, rational choice theorists, the paper notes that the Commission s room for manoeuvre is significantly reduced in co-decision, but it argues that its relative loss of power with the introduction of the procedure should not blur the picture that in absolute terms it is still an important actor in the day-to-day decision-making of the EU.