This paper investigates the dynamic and performative construction of publics in public engagement exercises. In this investigation, we, on the one hand, analyse how public engagement settings as political machineries frame particular kinds of roles and identities for the participating publics in relation to ‘the public at large’. On the other hand, we study how the participating citizens appropriate, resist and transform these roles and identities, and how they construct themselves and the participating group in relation to wider publics. The empirical basis of our argument is a discussion of four different kinds of participation events in Austria. Building on these observations we develop conclusions about the public up-take of public participation in technoscience and the role of public engagement in current techno-political cultures.