In this article, we explore the institutional negotiation of public engagement in matters of science and technology. We take the example of the Science in Society dialogue program initiated by the UK's Royal Society, but set this case within the wider experience of the public engagement activities of a range of charities, corporations, governmental departments, and scientific institutions. The novelty of the analysis lies in the linking of an account of the dialogue event and its outcomes to the values, practices, and imperatives-the institutional rationality-of the commissioning organization. We argue that the often tacit institutional construction of scientific citizenship is a critical, and relatively undeveloped, element of analysis-one that offers considerable insight into the practice and democratic implications of engaging publics in science and science policy. We also present evidence indicating that over time the expanding "capacities" associated with dialogue can act in subtle ways to enroll other elements of institutional architectures into more reflexive modes of thinking and acting. In the concluding section of the article, we consider the ways in which research and practice could (and we believe should) engage more squarely with facets of institutional context and culture.