This paper examines the proposition advanced by Sunstein (2001) and other scholars that political online forums tend to be characterized by in-group homogeneity and group polarization. The paper adopts a process view of online forums and examines discussions within a time perspective. Five discussion lines on Climategate.nl (a skeptical Dutch online forum on climate change) are investigated. The research focuses on how participants react to the participation of dissidents and on the resulting processes of inclusion and exclusion. Climategate.nl moved in the direction of an ‘echo chamber’ gradually over time. Nevertheless, the forum was never completely homogeneous. The editors played an active role in the inclusion and exclusion of dissidents. A counter-steering moderation policy is needed to keep group polarization and homogenization within certain limits.