This study determines whether an activity system provides a systematic framework to analyse collaborative group work. Using an activity system as a unit of analysis, the research examined learner behaviours, conflicting factors and facilitating factors while students engaged in collaborative work via asynchronous computer-mediated communication. The online postings from the two highest and lowest performing groups of students during one semester were analysed, coded and reorganised. The coded postings of these four groups were then compared using an activity system to identify the difference in patterns of learner behaviours. The conflicting and facilitating factors that students encountered were identified as well. The findings provide implications for online collaborative learning environment design and mediating strategies. The study also addresses some of the challenges in using an activity system to analyse collaborative learning activities and provides suggestions for future research.