Citizens are becoming increasingly likely to challenge the knowledge bases underlying policy programmes that deal with risks. This paper investigates how participants in online discussions engage in interactions between expert knowledge, ‘commons knowledge’ and policy assumptions. The concept of ‘boundary objects’, arrangements that allow different groups to work together without consensus, is used to analyse the role of online discussions in these interactions. Discussions on three Dutch online forums about the swine flu are investigated according to a framework for policy argumentation. Interaction between knowledge domains was limited, and it varied in focus and nature across the three forums. Each discussion functioned as a partial approximation of a boundary object. Government organizations should be more aware of the variety of online forums in which discussions about societal risks take place. Several practical options are presented for policy-making with regard to risks.