Students in competence-based vocational education are expected to actively construct a personal professional theory, in which they integrate different types of knowledge and beliefs. Students' personal professional theories are seen as an important learning outcome of competence-based vocational education. However, it is unknown how personal professional theories can be measured. This study focused on measuring the content and nature of students' personal professional theories using a multi-method triangulation approach, in which 16 students in the domain of Social Work constructed a concept map, an interview and a self-report. The results show that the relatively structured methods (i.e., interviews and concept maps) reveal more insight into students' personal professional theories than less structured methods (i.e., self-reports). It is concluded that both structure as well as adequate prompts are important in the process of explicating personal professional theories. (Contains 6 tables, 2 figures and 1 footnote.)