This paper details the design and implementation of a participatory modelling process in the Baixo Guadiana River Basin, in Portugal. A group of stakeholders was involved in a causal mapping exercise that lead to the development of a shared view of the problems, pressures and impacts characterizing the river basin. A simulation model was also developed to support experimentation with alternative management scenarios for the area. The paper looks critically at the evaluation of the participatory modelling outcomes, both at the individual and group levels, discussing the role of this approach in supporting the scoping stages of river basin planning and management processes. On the downside, the unstable group composition seems to have hindered the chances of producing a higher impact in the functioning of the group, and subsequently the capacity to sustain the level of collaboration required to achieve the strategic river basin objectives established during the participatory modelling process. The paper discusses the issue of group stability as well as some options to overcome the limitations of unstable participant groups. The strengths of participatory modelling, as underlined by the Baixo Guadiana experience, include the flexibility to adapt the method to different contexts and participatory designs and the capacity to structure the active involvement of stakeholders, providing an open and shared language for collaborative policy design, fostering learning and knowledge integration. (C) 2008 Elsevier B.V. All rights reserved.