Abstract This article analyses the roles and impact of Interinstitutional Agreements (IIAs) in the EU, taking into account their relationship to primary law. Concretely speaking, these roles range from (a) explicitly authorised specifications of Treaty provisions via (b) not explicitly authorised specifications of vague Treaty law to (c) pure political undertaking. Based on the distinction between the constitutional and the operational level of the political game, we challenge the assumption that IIAs usually strengthen the European Parliament. As our case study, the 1993 interrelated package of IIAs on democracy, transparency and subsidiarity, illustrates, the European Parliament is not the only institution that benefits from IIAs, especially if they lack a sufficiently precise Treaty basis. Furthermore, if Treaty provisions underlying IIAs are precise, they also tend to produce precise and thus legally relevant content. Conversely, if IIAs deal primarily with elusive concepts they are likely to be legally ambiguous or even irrelevant at all.