The objective of this paper is to investigate the determinants of European Union (EU) merger control decisions. We consider a sample of 167 EU mergers between 1990 and 2002 and evaluate their competitive consequences by the reaction of the stock market price of competitors to the merging firms. We then account for the discrepancies between the actual and optimal decisions as indicated by the stock market in terms of the political economy surrounding the cases. Our results suggest that the commission’s decisions cannot be solely accounted for as protecting consumer surplus. The institutional and political environment does matter. As far as influence is concerned, however, our data suggest that the commission’s decisions are not sensitive to firms’ interests. Instead, the evidence suggests that other factors—such as market definition and procedural aspects, as well as country and industry effects—do play a significant role.