Referendums have received increasing attention after the recent round of votes on the Maastricht treaty and the widening process of the European Union. Despite this increased interest in these instruments of decision-making, scholarship has not provided us with insights into the relationship between the institutional characteristics and voter's decision. We provide a theoretical argument on how the voter's choice is affected by the nature of the referendum. Relevant factors are whether the referendum is required, whether the people's decision has a binding character, or which government coalition is at the time in power. These institutional features mediate the impact of political factors, above all partisanship, on voting behavior. We test our theoretical arguments based on empirical material from fourteen referendums on European integration and find consistent support for our theoretical contentions.