Teaching science in the English language is becoming increasingly common around the world. Drawing on 34 h of video-recorded classroom observations of 19 teachers, I report a study of the teaching process in EMI science classrooms at 8 secondary schools in Hong Kong. I compared the patterns of classroom interactions (turn-taking, ratio of student to teacher talk, language choices, question types) in two types of EMI schools. The results of analyses of the observational data show similar interactional patterns in the early–full and late–partial EMI science classrooms when measured as percentages of interaction time, distribution of speaking time of teachers and students, and frequency of pedagogical functions. However, the nature of the interactions differs. In the late–partial EMI schools, there are generally larger numbers of student initiations and responses, but these initiations and responses are shorter in duration. Additionally, teachers in late–partial EMI schools use more higher-order questions but offer less direct feedback to students. Teachers and students in both types of schools tend to use their L1 more often than their L2. By providing an evidence-based, detailed analysis of classroom interactions, this research sheds light on ways to improve instructional practices in EMI classrooms worldwide.