Feedback is an essential formative assessment practice that has the potential to influence student learning and achievement positively. Providing effective feedback, however, is a challenging task for teachers. Especially beginning teachers struggle with the provision of information that supports students in developing and improving their competences. Learning to provide feedback thus is an important aspect of teacher education. The present exploratory study investigates pre-service chemistry teachers' practices regarding the judgement of students' level of achievement and the provision of feedback in the context of the control-of-variables strategy. A sample of N = 40 bachelor and master students at Kiel University judged students' written artefacts and provided feedback with the aim to support students in their progression towards the learning goals. The quality of the feedback was coded with respect to its correct judgement of the students' current levels of achievement and the support it provided regarding next steps in learning. The results show that only a few of the pre-service teachers could correctly judge students' current levels of achievement and provide feedback that is expected to be effective. Overall, the findings indicate a need for more research in this field and a need for the implementation of formative assessment practices as a topic in pre-service teacher education.