This paper describes a study of whether and how collaborative learning at the computer fosters conceptual change. A suite of computer simulation programs developed to confront students' alternative conceptions in mechanics was integrated into a grade 10 science class in a high school in Melbourne, Australia. A Conceptual Test was administered as a pre-, post-, and delayed post-test to determine students' conceptual change. Students worked collaboratively in pairs on the programs, carrying out predict-observe-explain tasks according to a set of worksheets. Conversational interactions of seven pairs in the class were analyzed for mode of collaboration by joint on-task engagement and by equality and mutuality of engagement. Collaborative sequences were categorized as co-constructions of shared understanding or peer conflicts. Inferences were made as to whether these led to conceptual change. Computer-supported collaborative learning provided students with experiences of co-constructions of shared understanding and peer conflicts, which led to conceptual change for those who were cognitively engaged in the tasks and prepared to reflect on and reconstruct their conceptions. To achieve long-term and stable conceptual change, this should be accompanied by students' personal construction and sense-making of the new understanding. Contains 43 references. (Author/PVD)