This article investigates the effect of participation in synchronous online discussions on the development of individual reasoning as manifested in reflective essays. Students from a fourth-grade classroom engaged in a series of real-time online discussions. The format of the discussion was based on the Collaborative Reasoning approach which has clear guidelines and procedures. The results showed that compared to the students from the contrast classroom, the students who had experienced synchronous online discussions wrote essays with more satisfactory arguments, counterarguments, rebuttals, and textual information. In addition, they wrote longer essays which contained fewer irrelevant statements. These effects of online discussions were consistent between girls and boys and between shy and non-shy children. The students' uses of arguments and rebuttals, as well as the length of their essays, were adequately predicted by the frequency of their contributions in the discussions. The present study suggests that when an online chat is implemented with an instructional approach which has well-established procedures and effective moves, it can provide an intellectually stimulating context in which "all" students can learn to reason by interacting with one another.