Internet users increasingly interact with others through these new and advanced forms of online communication. Many of these interactions involve complex reasoning processes. Compared to face-to-face communications, participation in Open Online Communications is anonymous, asynchronous, and open to any Internet user or registered community member. In addition, participants can choose when to join and when to leave the communication. On the other hand, participants may have heterogeneous demographic information (e.g., age), and varied domain expertise and professional background. With these characteristics in Open Online Communications (OOC), Internet users may find it particularly challenging to identify, interpret, and evaluate the others’ ideas and opinions. I will talk about a few research projects for understanding and supporting Internet users’ reasoning processes. These projects focus on two aspects: 1). Internet users’ rationales and their rationale awareness, and 2). Online persuasion study.