Trying to understand the complexity of computer-mediated problem-based learning environments is not easy. Sociocultural theory provides a theoretical framework for understanding such environments because it emphasizes the socially situated nature of learning and the critical role of tools in mediating learning. To examine how different aspects of discourse relate to each other, as well as to the tools being used in the collaborative learning process, it is important to understand how collaborative knowledge building unfolds and how these processes are mediated. This requires going beyond coding individual speech acts. The use of Chronologically-Ordered Representations of Discourse and Tool-Related Activity (CORDTRA) diagrams is one way of achieving this understanding. We use this to study contrasting cases of more and less successful groups using the STELLAR learning environment. STELLAR is an integrated online PBL environment for preservice teachers, containing a learning sciences hypermedia, a library of video cases, and online personal and collaborative spaces. Our analyses suggest that an important locus of differences is how students use resources and engage in different kinds of metacognitive talk and knowledge transforming activities, sometimes to the group's detriment. Frequency analyses provided an easily interpreted snapshot of each group's activity. The CORDTRA analyses provide a more dynamic view that helps researchers and teachers better understand how collaborative learning unfolds. Such analyses have implications for understanding new learning environments as well as helping identify where interventions might be needed.