Purpose – 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 aims to address this issue. Design/methodology/approach – The paper describes 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. Findings – Results from the analysis illustrate participants’ 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 systems as a holistic and flexibleunit of analysis for studying CKB is illustrated through discussion of the results. Research limitations/implications – The results have educational research implications in terms of developing research tools for analysing CKB, collecting data from a group context, and developing tools for improving group-work. Practical implications – The results have practical implications in terms of building knowledge from experience within knowledge communities. Originality/value – One of the outcomes of the study is the identification of developmental and reflective contradictions which highlight the issues that when addressed allow for successful achievement of the object (or to some degree of success), as well as a richer deeper experience for the participants.