In competence-based vocational education, personal professional theories, in which students integrate different types of knowledge and beliefs, are seen as important. Exactly how these theories can be measured is the main focus of this study, which uses a multi-method triangulation approach, an interview and a self-report. The latter (less-structured) matter seems to provide less insight into personal professional theories then the structured methods. Both structure and adequate prompts are important when personal professional theories are explicated.