In this article we consider how the roles of learner and teacher are configured in a teletandem exchange between two students working in a university language centre with no external tutor present. Teletandem is a multilateral exchange (Lewis et al. 2011) via desktop videoconferencing. It is based on the tandem method, where two learners with different mother tongues learn each other's language through multimodal online interaction. It relies on learner autonomy and reciprocity, which means that every learner is supposed to be half of the time learner of one language and half of the time expert in the other. Our study shows that teletandem partners assume expert and learner roles not only in relation to the language spoken, but also according to the topic discussed, generating what we call "crossed expertise". Moreover, we identify different degrees in the use of multimodality. These findings help us to understand how roles are constructed in a self-guided language learning environment and in this way contribute to the improvement of tandem learning arrangements and the design of tandem learning environments.