Tutors often have difficulty in accurately assessing their tutees’ understanding. However, it is unclear how tutors’ professional experience influences their assessment accuracy. Therefore, we conducted a study with N = 46 tutor-tutee dyads and compared the accuracy with which teacher tutors and student tutors assessed a tutee’s understanding of the human circulatory system at the level of mental models. Results showed that both groups of tutors faced similar difficulties in assessing a tutee’s understanding. However, whereas teacher tutors’ assessment accuracy remained constant in the course of tutoring, student tutors’ assessment accuracy decreased. Moreover, teacher tutors more accurately self-assessed their assessment accuracy than student tutors. Although teacher tutors process diagnostic information more accurately than student tutors, both groups of tutors seem to be overwhelmed by processing all information making up a tutee’s mental model. Hence, regardless of their professional experience, tutors need to be supported in assessing a tutee’s understanding.