Based on a purposive sample of 15 second-year chemical engineering students, this study investigates students' conceptions of evaporation, condensation and vapour pressure. During individual interviews the students were questioned on three tasks that had been designed around these topics. Qualitative analysis of student responses showed a range of conceptions in each area, including some misconceptions. A key underlying misconception was the belief that evaporation and condensation require a temperature gradient in order to take place. Many students changed their initial (incorrect) answers when presented with further physical evidence as the interview progressed. The study points to the importance not only of practical work, but of associated conceptual discussions that allow students to reflect on and refine their conceptions.