Students in competence-based senior secondary vocational education are expected to actively construct personal knowledge during school-based learning and workplace learning. This study introduces the construct "personal professional theory" (PPT) to monitor and analyse students' personal knowledge development. It aims to investigate the development of the content and nature of students' PPTs and their perceptions of learning processes in vocational education during school-based learning and workplace learning. One group of students (n=20) in the domain of Information and Communication Technologies (ICT) is followed during one course of 40 weeks in which they alternately participate in school-based learning and workplace learning. A longitudinal design (t=4) including semi-structured concept maps is used to reveal the content and nature of students' PPTs (n=240). Semi-structured interviews (n=10) and log files (n=39) are used to explore students' perceptions of their learning process. No significant differences are found between the four measurements concerning the content and nature of students' PPTs. However, the actual content of students' PPTs shows a relatively unsteady and changeable pattern. Additionally, students perceive workplace learning as an essential process for the development of their PPT. It is concluded that the development of students' PPTs involves knowledge compilation as well as thematic turbulence. (Contains 5 tables and 2 figures.)