This paper describes a model for analysing collaborative knowledge building (CKB) process during a group activity. Singh et al’s (2007) model of CKB process is used for analysing the process using data from an educational case. The concept of CKB activity system is developed based on the constructs of activity theory for analysing the case. The analysis of the case used constructs from reflective thinking to highlight the processes used by participants to collaboratively build knowledge. The findings of the study identified four additional cycles of reflective thinking activities that have been used to modify the CKB process model. The four additional cycles represent the mediating processes and tools used by the participants to build knowledge. The modified model of a CKB process together with the CKB activity system can be used as the unit of analysis for developing a cohesive theory for understanding and analysing CKB.