This paper reports on an exploration into difficulties in junior secondary science lessons taught through English in the largely Cantonese-speaking context of Hong Kong. The study was carried out by an educational linguist and a science education specialist working in collaboration, and identified difficulties through student questionnaires, interviews with teachers, and classroom observation. Understandings of genre based in systemic functional linguistics were used in conjunction with knowledge of the science curriculum to investigate these difficulties. Extracts from lessons and teaching materials on energy conversion are discussed, with additional reference to interviews and questionnaires. Analysis revealed the influence of teaching materials on coherence between curricular objectives and teacher discourse. In particular, complex description detracted from explanations, affecting student understanding of the lesson. Copyright ©2024 Elsevier.