Wikis as shared digital artifacts may enable users to participate in processes of knowledge building. To what extent and with which quality knowledge building can take place is assumed to depend on the interrelation between people's prior knowledge and the information available in a wiki. In two experimental studies we examined the impact on learning and knowledge building of the redundancy (Study 1) and polarity (Study 2) between participants' prior knowledge and information available in the wiki. Based on the co-evolution model of cognitive and social systems, external assimilation and accommodation were used as dependent variables to measure knowledge building. The results supported the hypotheses that a medium level of redundancy and a high level of polarity foster external accommodation processes. External assimilation was stimulated by low redundancy and a high level of polarity. Moreover, we found that individual learning was influenced by the degree of external assimilation. (Contains 4 figures.)