The elaborate system of expert groups that the European Commission organises is a key feature of EU everyday governance and also a potential channel of societal involvement in EU policy making. This article examines the patterns of participation in the expert group system of a broad set of societal actors-NGOs, social partners/unions, consumer organisations, and business/enterprise. The analysis is based on a large-N study of Commission expert groups. Taking on an "executive politics" perspective, we identify main patterns of participation and analyse organisational factors that affect the inclusion of societal actors in the expert group system. We find that such actors are strongly involved in this system. Yet, there is a striking heterogeneity in the extent to which the Commission's administrative units include societal groups as experts in the policy process. The logics that underpin the inclusion of business organisations are not identical to the logics of inclusion applied to social partners and NGOs. The Commission as the core supranational executive is thus selectively open for societal involvement in its expert groups system, and this bureaucratic openness is patterned, clustered, and conditioned by structural factors that affect how the Commission as a multi-organisation operates.