Currently, numerous on-line discussion forums have been developed for educational purposes; therefore, a better understanding about peer student discussion or discourse interactions is quite important. Through gathering peer learning interactions on 57 college computer science students, who were randomly assigned into 14 small groups for solving programming problems, this study analyzed the content of their discourse interactions. First, it was revealed that the most frequent interactions were related to some questions or suggestions regarding how to effectively coordinate peer members. However, different features of peer interactions were found across different small groups. By time sequence analysis of peer interactions, it was found that issues and positions were proposed mostly in the initial and middle stages of the study, while the conflicts frequently occurred in the beginning stage. Finally, this study suggested five peer interaction patterns in terms of peer knowledge exchange, including centralized knowledge exchange, distributive knowledge exchange, group development impediment, ability impediment and partial knowledge exchange. A further analysis of students' knowledge exchange patterns revealed that peer students' background abilities played an important role on the ways of knowledge exchange involved in the on-line peer learning activity. Certain configurations of students' background abilities tended to lead to a particular communication pattern. The implications derived from the findings for educational practice were also discussed. For example, the small groups with peer members of high achievement (or heterogeneous abilities) may not guarantee the success of group work. Many of them need teachers or moderators to scaffold the process of peer interactions and learning.