The rise of English-medium education has led to considerable academic interest in communicative practices in multilingual university settings. However, there is still little research into disciplinary knowledge construction in higher education contexts where English is the academic lingua franca. To address this gap, we embrace the central role of explaining in education by proposing Exploratory Interactive Explaining (EXINTEX) as an analytical tool, which identifies the elements of explanatory episodes in the joint development of disciplinary knowledge. We then apply this tool at the micro-level of interaction to two comparable contexts differentiated primarily by the presence or absence of a teacher. Our findings confirm the relevance and high level of educational success of EXINTEX episodes in both quantitative and qualitative terms. They further reveal the dynamic interplay of epistemic authority and engagement in the learning process. Additionally, they show how research into educational disciplinary discourse and English as a lingua franca in academic settings can complement and supplement each other.