In the three latest treaty reforms changes have been made to the procedures that regulate the legislative interaction between the European Parliament, the Commission and the Council. Whether the introduction of the co-operation procedure (1987), the co-decision procedure (1994) and the reform of the co-decision procedure (1999) have marked a linear increase in the power of the EP has been the object of debate. However, it has not been disputed that these three procedures vis-à-vis the consultation procedure give the EP a significant legislative role not previously enjoyed. To explain the empowering of the EP, this paper takes its point of departure in the causality between institutional choices and institutional consequences. Using spatial theory and drawing on historical institutionalism it introduces an analytical model that operates on two levels, the level of day-to-day politics and the level of treaty reforms. Two main points are made. First, adapting strategically to the de jure legislative procedures the EP has been able to convert these into de facto procedures that grant it a substantial legislative role. Second, this empowering has enabled the EP and the Commission to pursue a pro-integrationist agenda in a two-level game of legislation and implementation.