We examine the sources and processes of institutional change in one important aspect of EU politics-the legislative procedure of codecision and show how interstitial change of institutions emerges between formal Treaty revisions and under specific conditions may be formalized in subsequent formal Treaty reforms. We develop two related models of Treaty change. First, in a 'simple' model, we argue that informal rules will be formalized in the Treaty text where all member states are in agreement, and will be rolled back when all member states oppose them; otherwise they will continue in existence at the informal level. Second, in a more complex framework, we argue that actors who have effective veto powers in a related arena may make credible threats that allow them to press member states into formalizing informal rules, provided that member states are not unanimously opposed to this formalization. We empirically assess our claims in the light of several instances of informal rules applied in the codecision procedure.
