This study investigated the effect of well- vs. ill-structured problem types on: (a) group interactional activity, (b) evolution of group participation inequities, (c) group discussion quality, and (d) group performance in a synchronous, computer-supported collaborative learning (CSCL) environment. Participants were 60 11th-grade science students working in three-member groups (triads) who were randomly assigned to solve a well- or an ill-structured problem scenario on Newtonian Kinematics. Although groups solving ill-structured problems generated more problem-centered interactional activity (a positive effect), they also exhibited participation patterns that were more inequitable (a negative effect) than groups solving well-structured problems. Interestingly, inequities in member participation patterns exhibited a high sensitivity to initial exchange and tended to get "locked-in" early in the discussion, ultimately lowering the quality of discussion and, in turn, the group performance. Findings and their implications for theory, methodology, and scaffolding of CSCL groups are discussed.