In this paper, the potential of synchronous net-based learning discourses as a special case of computer supported cooperative learning (CSCL) is analysed. Discourses among learners, or among learners and tutors, can significantly improve understanding of complex subject matter. However, net-based discourses are often suboptimal, since technical restrictions constitute barriers to an efficient exchange of knowledge. Support of net-based discourses may proceed either on the macrolevel of the overall discourse structure, or on the microlevel of single contributions. As a central microlevel activity the grounding of contributions is identified ...