This study aimed to provide an account of how learning takes place in problem-based learning (PBL), and to identify the relationships between the learning-oriented activities of students with their learning outcomes. First, the verbal interactions and computer resources studied by nine students for an entire PBL cycle were recorded. The relevant concepts articulated and studied individually while working on the problem-at-hand were identified as units of analysis and counted to demonstrate the growth in concepts acquired over the PBL cycle. We identified two distinct phases in the process--an initial concept articulation, and a later concept repetition phase. To overcome the sample-size limitations of the first study, we analyzed the verbal interactions of, and resources studied, by another 35 students in an entire PBL cycle using structural equation modeling. Results show that students' verbal contributions during the problem analysis phase strongly influenced their verbal contributions during self-directed learning and reporting phases. Verbal contributions and individual study influenced similarly the contributions during the reporting phase. Increased verbalizations of concepts during the reporting phase also led to higher achievement. We found that collaborative learning is significant in the PBL process, and may be more important than individual study in determining students' achievement.