English-medium instruction (EMI) as a curricular strategy for improving the quality of higher education and internationalizing Chinese universities has enjoyed strong policy support. The co-presence of different mediums of instruction in the same lesson created a naturalistic, ideal research design to investigate the effects of instructional medium on the types of teacher questions and student responses posed, as well as their cognitive and syntactic complexity. All the teacher questions and student responses in the transcribed classroom discourse were coded for cognitive and syntactic complexity. Following T. Lynch and K. Wu, a teacher question was defined as an interrogative, imperative, or declarative utterance which elicited a content- or language-related verbal response from a classroom participant, including the questioner himself or herself. One-way within-subjects ANOVAs were run to determine the effect of instructional languages on the cognitive and syntactic complexity of teacher questions.