Research on legislative voting behaviour in the European Parliament (EP) has largely examined group cohesion by analyzing why MEPs vote against their group lines, with typical explanations emphasizing electoral rules and legislative agenda-setting. This paper takes a step forward in current research in several respects. First, it combines group defection to political groups and support to the final legislative outcome as a dependent variable. And second, it builds on and combines previous literatures on voting behaviour and inter-institutional relations in EU policy-making, and tests hypotheses on both individual legislators’ properties and the institutional characteristics of ‘codecision’ processes. The analysis is based on a novel dyadic datset on MEPs behaviour on all final passage votes during the seventh and first half of the eighth legislatures. The paper is expected to make a contribution to the literature on legislative behaviour in the EP.