This article examines political party behaviour around the referendums on the EU Constitutional Treaty in 2005. Starting from the presumption that this behaviour needs to be analysed in the light of the domestic government-opposition dynamics, a set of hypotheses on the causes and consequences of party behaviour in EU Treaty referendums is developed and reviewed for the EU member states in which a referendum was held or anticipated. As it turns out, with the exception of some right-conservative parties, all mainstream parties endorsed the Constitutional Treaty. However, because significant proportions of opposition party supporters are bound to go to the 'No' side, government parties are eventually crucial in securing a majority in favour of EU Treaty revisions. Copyright © 2007 Sage Publications.