Abstract: Instant messaging has recently gained the attention of educational researchers who are investigating how this medium may promote learning through collaboration, increased participation and interaction. While several studies indicate that IM has great potential, there is scant evidence as to whether a synchronous tool such as instant messaging can support high-order learning, such as critical thinking. This paper describes a pilot study that attempted to detect critical thinking in synchronous online discussions mediated by an instant messaging tool. The study involved 22 graduate students from a university in Western Canada. Eleven transcripts (containing over 700 messages) were analyzed based on the four cyclical phases of the practical inquiry model (PI). Preliminary results indicate that when IM interaction is planned and structured, it has the potential to support and sustain critical thinking in online discussions.