A knowledge about pedagogical diagnosis is important for professional teachers for teaching heterogeneous chemistry classes. For chemistry teachers, the knowledge about their students` perceptions, attitudes, knowledge, previous experience or their interest is crucial for planning lessons. Therefore, the diagnosis is a key component of teachers` knowledge. However, research in this area is still underrepresented. It is clear, that the development of teachers` knowledge about diagnosis in chemistry teaching and learning should be started during the university teacher education program. But, how does this knowledge develop or change during the teacher training program in chemistry education?
This paper attempts to investigate this development with a longitudinal interview case study. Two chemistry student teachers participated the study and were interviewed at seven different time points during their teacher education program. The interview and the analysis are based on the definition by Jäger with a focus on the four dimensions of the diagnostic competence: i. Competence Knowledge, ii. Conditional Knowledge, iii. Technological Knowledge and iv. Knowledge of Change. The analysis of the interviews follows documentary method. The results show a different development of student teachers, which allowed a contrasting view of the students. From this, implications for the teacher training will be presented.