Synchronous written chat and instant messaging are tools which have been used and explored in online language learning settings for at least two decades. Research literature has shown that such tools give second language (L2) learners opportunities for language learning, e.g. , the interaction in real time with peers and native speakers, the written mode of language, and the time available for planning and monitoring utterances. However, since the majority of the empirical work on chat and instant messaging has been conducted under experimental conditions, relatively little research has investigated how interaction in chat influences language learning opportunities under the conditions of an online academic language course where students have unequal status because of their different language background and level of L2 proficiency. This article presents an explorative study of the interaction in chat in a web-based academic language course between students with different L1s and different levels of L2. The aim is to shed light on how student interaction in an institutional context benefits the language learning environment in a manner that promotes L2 learners' attention to linguistic items in their input and output, and that allow opportunities for functional practising. Based on a mainly quantitative analysis, this article illustrates how L2 learners' participation, the complexity of their utterances, and their opportunities for self-correction and corrective feedback are influenced by group formation.