AbstractScience writing has played a crucial part in science assessments. This paper reports a study in an area that has received little research attention – how science lessons in content and language integrated learning (CLIL) can increase the science knowledge development of English as a foreign language (EFL) students in Hong Kong. The data come from a school-based interventional study in chemistry classrooms, with written data from questionnaires, assessments and teachers’ logs and verbal data from interviews and classroom observations. The effectiveness of the CLIL teaching and learning activities in various chemistry classrooms were compared and evaluated, with a discussion of some implications. The paper concludes that CLIL teaching and learning activities yielded positive learning outcomes among chemistry learners with low English ability.