This paper models the correlates of parties' positions on the issue of European integration, asking why some parties are in favour of European integration, while others are less favourable or even opposed to it. The paper builds on existing work which has identified three sets of explanatory factors predicting parties' positions on integration: the electorate, parties and party system characteristics. By employing multilevel modelling using data on over 220 parties in 14 Western EU member states for the years 1984 to 2006, the effects of party- and context-level predictors of parties' positions on EU integration are assessed. The findings demonstrate that parties' positions are primarily influenced by EU preferences of the general electorate, parties' left-right ideological extremes and incumbency status. The results also show that the impact of party characteristics is moderated by the electoral context in which parties operate. Moreover, the interaction between both levels offers further insights as to the nature of these associations. Specifically, party size is a robust predictor of integration position only when accounting for the levels of party system's fractionalisation and polarisation. Additionally, parties oriented towards the centre of the ideological spectrum are even more likely to favour European integration within highly polarised systems.