Computer-supported learning and knowledge building play an increasing role in online collaboration. This paper outlines some theories concerning the interplay between individual processes of learning and collaborative processes of knowledge building. In particular, it describes the co-evolution model that attempts to examine processes of learning and knowledge building by working on wikis. We report an experimental study that aimed at testing some predictions of this model empirically. The results support the assumption that accommodative knowledge building and a development of conceptual knowledge takes place particularly when there is incongruity at a medium level between people's knowledge and the information contained in a digital artefact. In contrast, assimilative knowledge building and the development of factual knowledge depends largely on people's prior knowledge. Concluding, the consequences of these findings on educational uses of wikis are discussed. (Contains 3 figures.)