Collaborative knowledge building (CKB) is seen as a means for achieving desired learning outcomes as well as facilitating sharing and distribution of knowledge among community members. However existing CKB studies do not appear to identify and account for the tools used by groups (at individual and group level) as part of the CKB process. The paper addresses this issue by describing a group knowledge building exercise within an educational context using activity theory as a descriptive data analysis tool. Data analysis involved conceptualising the CKB process as an activity system in which the group worked towards a shared object and identifying the ensuing contradictions in the CKB activity system. Results from the analysis illustrate participant’s use of reflective thinking processes for resolving contradictions and as a tool for articulating knowledge and developing a shared understanding. Two types of contradictions are identified from the analysis resolving which helped the group to achieve their objective. The efficacy of using activity system’s as a holistic and flexible unit of analysis for studying CKB is illustrated through the discussion of results. The results have educational as well as practical implications in terms of developing research tools for analysing CKB, collecting data from a group context, developing tools for improving group-work and building knowledge from experience within knowledge communities.