Placement mentors' role increasingly implies demonstrating to student teachers how research-based knowledge in combination with experience-based knowledge may be relevant in teachers' professional work. This is a challenge. Placement mentors are often unsure how to make sense of research-based knowledge. Frequently there is a mismatch between what they say they can do and what they actually show they are able to do. This paper explores how placement mentors' reasoning is formed by their lack of power to define what research-based knowledge consists of. The analysis in this paper is based on an investigation of the epistemological premises that placement mentors rely on when they validate research-based knowledge. The theoretical-analytical point of departure is Michel Foucault's conception of power-knowledge.