This paper reports on a project in which a group support system (GSS) equipped with a causal mapping facility was used to acquire knowledge from experts in seven European cities in order to understand the systemicity of risks which cities may face. The practical constraints demanded that participants’ experience and wisdom about the city risk environment was collected in a short period of time: three 1-day workshops. The acquisition of knowledge posed a number of important epistemological challenges which are explored in our discussion. The GSS was faced with the need to (1) facilitate sharing of knowledge with others, (2) manage the complexity of expert knowledge, (3) acknowledge the time demands on experts, (4) manage and merge multiple perspectives, and (5) acknowledge the subjectivity of knowledge in this domain. By discussing how the GSS process attended directly to these epistemological issues and to methodological considerations that linked to these issues, the paper contributes to a better understanding of the application of GSS for knowledge acquisition, particularly in comparison with other possible methods.