In this study, the effectiveness of secondary education teachers' interpersonal behaviour is investigated by analysing data from 2 samples: a study on 45 Physics teachers and their 3rd-year classes and a study on 32 English as a Foreign Language (EFL) teachers and their 3rd-year classes. Teacher interpersonal behaviour was studied by means of students' perceptions of this behaviour, collected with the Questionnaire on Teacher Interaction (QTI). These perceptions include 2 important dimensions: Influence and Proximity. Results of multilevel analyses with various covariates indicated that Influence and Proximity were positively related to both cognitive and affective outcomes. Interpersonal behaviour explained up to more than half of the variance in student outcomes at the teacher-class level. The outcomes suggest that interpersonal behaviour as perceived by students may be an important variable for educational effectiveness researchers.