<p>The aim of this study was to explore the interconnections between communicative activities, situational stress reactions, and cognitive engagement in undergraduate mathematics lessons. The multimodal data consisted of video recordings of the lesson and cardiac physiological measurements of students. Whereas situational stress levels sustained higher during individual students’ presentation turns, in-depth moment-by-moment analysis of selected critical incidents showed the co-occurrence of situational stress reactions and specific communicative activities. The results showed that distinct changes in situational stress co-occurred with conflict situations when presenting and communicating mathematical problems. On the other hand, as depicted in both observations of behaviour as well as changes in situational stress, cognitive engagement favouring learning was considered to take place because of peer interaction leading to individual resolving of an incorrect solution. Implications for educational research and mathematics education are discussed.</p>