A previous study highlighted the perception among secondary science teachers that they faced considerable challenges to their pedagogical practice when teaching unfamiliar areas of the curriculum; for example, when teaching out of subject specialism. One of the major challenges cited by the teachers was being able to give appropriate and effective science teaching explanations in the classroom. Since talking in order to explain science is at the centre of what science teachers do, this concern is a significant one for teacher educators. This article considers some of the methodological issues about how to investigate the relationship between teachers' subject content knowledge and their pedagogical practice. The research outlined focuses on a single science teacher's practice in giving science teaching explanations when teaching in and out of subject specialism. Although the findings from a single case are of limited value in terms of generalisability, this study adds to the discussion about future research into the relationship between teachers' professional knowledge bases and their pedagogical practices. The following are appended: (1) Descriptive analysis of the 10 lessons; and (2) Exemplification of the descriptive analysis of explanatory episodes. (Contains 4 figures.)