An exploratory study of students' engagement in online learning and knowledge building is presented in this paper. Learning in an online community, composed of students (pre-service teachers) and experts (experienced in-service schoolteachers and academics), is the study's primary focus. Students' interaction and knowledge discourse structures, arising from individual readings of academic papers and asynchronous collaboration with peers and experts, are investigated using social network and content analysis techniques. Additionally, several new measures for exploring structural-qualitative aspects of knowledge discourse are introduced. Analysis revealed several important trends. First, students' interaction was more intensive in forums where experienced teachers participated, rather than students only. Second, students' individual discourse structures in their postings were quite deep, knowledge-focussed and elaborated; while students' replies were short, usually focussed on specific idea and contained a substantial amount of non-cognitive information. Overall, it is argued that students were engaged with the individual and collaborative knowledge building in the online learning community. Practical implications of the study results for development of courses are discussed.