The purpose of this study was to explore the development over time of students' understandings of the concept of chemical reaction in the context of two familiar reactions in solution. The study is based on interviews of 48 students, aged 16–18, who had been successful in their year 11 examinations and had selected to study chemistry as one of their advanced level subjects in year 12 (age 16–17) and year 13 (age 17–18). All the students were interviewed (on a one-to-one basis) twice, once in year 12 and again in year 13, allowing progression in their understanding to be studied. The results show that students made some progress in their understanding of the concept of chemical reaction, but some fundamental misconceptions remained. © 2001 John Wiley & Sons, Inc. Sci Ed85:568–585, 2001.