Following a model on the cyclical nature of teacher ("trait") self-efficacy and context-, task- and situation-specific ("state") "mastery experiences" (TSSME), we investigated the variability and effects of lesson characteristics (e.g. lesson sequence), student group characteristics (e.g. proportion of students receiving free school meals) and teacher characteristics (e.g. teacher experience) on teachers' situation-specific mastery experience. Forty-three teachers reported on 1,055 lessons in 385 student groups using electronic questionnaires in Personal Digital Assistants during a period of 2 weeks. Two domains of TSSME (support of learning and organisation of classrooms) and perception of students (engagement and behaviour) were found. Multilevel models found roughly a quarter of the variance in TSSME between teachers, a quarter between student groups and half between lessons. Student group characteristics differentially predicted TSSME. Perceived student engagement was more predictive of TSSME than perceived student behaviour. More experienced and high-efficacy teachers had higher TSSME. The findings have implications for our understanding of teachers' everyday practice with different student groups.