A cascade model of professional development presents a particular risk that "knowledge" promoted in a programme will be diluted or distorted as it passes from originators of the programme to local trainers and then to the target teachers. Careful monitoring of trainers' and teachers' knowledge as it is transferred through the system is therefore imperative. This paper focuses on the transfer of content knowledge through an in-service teacher professional development programme and offers an innovative methodology for investigating knowledge transfer, i.e. through insights gained during a mentoring process. The findings suggest that this methodology facilitated assessment of knowledge transfer because it involved the identification of knowledge in practice. The focus on knowledge in practice appeared to avoid a deficit model of trainers'/teachers' knowledge and revealed that content knowledge was generally being successfully transferred throughout the system. A detailed analysis of different aspects of content knowledge transfer suggested various foci for additional training.