Political parties are often conceptualized as unitary actors that have consistent preferences. This ‘hidden assumption’ often turns out to overlook heterogeneity within parties and, therefore, intra-party dynamics in explaining attitudes. Concerning devolution and federalisation, parties or MP's belonging to the same region are also often implicitly considered as having homogeneous viewpoints and attitudes. Relying on an original MPs survey carried out during the Belgian political gridlock of 2010-2011, this article uncovers some of the key dimensions of the intra-party dynamics through the analysis of MPs’ preferences towards institutional reform in Belgium. Far from being explained by party or community lines, our results demonstrate how MPs’ political and sociological background, national/regional identity, political career and inter-community relations strongly shape their preferences.