Widespread use of mobile instant messaging (MIM) both formally by educators and voluntarily by students to support coursework has resulted in increased interest in the academic utility of the technology. However, due to the relative newness of the technology there is a call for more studies on how MIM impacts teaching and learning. To address this need, this case study investigates how MIM mediates online knowledge construction within the context of an economics course project. Online interactions over the course of a 16-week semester were coded and analysed using the Interaction Analysis Model. Analysis of 4,685 online messages revealed evidence of all five phases of knowledge construction: the sharing of information, identification of dissonance, negotiation of meaning, testing, and collective understanding. Both the structure of the course project and the affordances of connectivity, context-free access, and quasi-synchronous communication were found to be key factors in facilitating the knowledge construction process. Findings indicate that MIM may be effective supporting online collaborative learning. Furthermore, specific examples of student interactions offer insights into the complexity of online discussion possible via MIM and the interrelationships between the various phases of online knowledge construction.