The relationships between European institutions and interest organizations differ from those of comparable administrations in the extent of reliance upon them for both output and input legitimacy. The European Commission’s roles on policy formulation, implementation, and monitoring make for the most important relationship among EU institutions and interest organizations, in which its use of interest groups as political supporters and messengers is also noteworthy. The Commission has developed a system over the past decade to structure its relationship with ‘interested parties’ in which two-way information flows are central. Does detailed scrutiny of the procedures involved find information-exchange regimes designed (the ‘constitutive politics’) and operationalized (the ‘operational politics’) to maximize benefits in favour of the Commission? To what extent does the emergence of rules and the ways in which these are translated into practice reflect changing goals by the European Commission of its information-exchange regime? These questions are answered by identifying and assessing the changing constitutive and operational politics of the main procedural regimes involved: impact assessments, together with their embedded consultation component as well as procedures for the use of expertise; and transparency-related regimes of access to information, as well as the Transparency Register of ‘interested parties’.