This study investigated the effects of visualization of participation during computer-supported collaborative learning (CSCL). It is hypothesized that visualization of participation could contribute to successful CSCL. A CSCL-environment was augmented with the Participation Tool (PT). The PT visualizes how much each group member contributes to his or her group's online communication. Using a posttest-only design with a treatment (N=52) and a control group (N=17), it was examined whether students with access to the PT participated more and more equally during collaboration, reported higher awareness of group processes and activities, collaborated differently, and performed better than students without access to the PT. The results show that students used the PT quite intensively. Furthermore, compared to control group students, treatment group students participated more and engaged more in coordination and regulation of social activities during collaboration by sending more statements that addressed the planning of social activities. However, equality of participation, awareness of group processes and quality of the group products was not higher in the treatment condition. Still, the results of this study demonstrate that visualization of participation can contribute to successful CSCL.