Providing instructional explanations is a central skill of teachers. Using interactive simulations, we examined the explaining skills of 48 prospective teachers attending a teacher education program for accounting in vocational schools in Germany. We used a performance-based assessment that relies on explanatory quality as an indicator of teacher candidates' explaining skills. Video analysis was used to assess the quality of prepared and impromptu explanations in respect of different quality aspects. We found that the prepared explanations of prospective teachers were of high quality in terms of "student-teacher interaction" and "language." With respect to the quality of "content" (e.g., accuracy, multiple approaches to explaining) and "representation" (e.g., visualization, examples), prospective teachers performed significantly worse. The quality of teacher candidates' improvised explanations was significantly lower. This was especially true for the quality of "representations," the "process structure," and the interaction between student and teacher. For four of the five quality criteria examined, no correlation could be found between the quality of prepared and improvised explanations. For the "language" criterion, however, there was a correlation between the two types of explaining situations. Implications on how to support teacher candidates in developing explaining skills during teacher education are discussed.