Epistemic beliefs are individual beliefs about knowledge and knowledge acquisition. Empirical studies indicate that learners' epistemic beliefs influence their learning processes and success (e.g. motivation, text comprehension, learning strategy selection, grades). Teachers and trainers can support their apprentices in developing preferable epistemic beliefs to facilitate learning. To do this, instructors need information on learners' current beliefs for lesson planning and monitoring epistemic beliefs development. The following paper reports the validation of a tool for generating this kind of information: the Instrument for Measuring Epistemic Beliefs in Marketing. The instrument validation is based on three studies (Study I: 225 university trained business administration students; Study II: 531 vocationally trained retailers/wholesalers; Study III: 179 vocationally trained bank assistants). It reveals high reliabilities (a values about 0.700), stable factor structures, and incremental validity in comparison to general epistemic beliefs. The new questionnaire is able to predict grades in marketing and can be used to diagnose apprentices' epistemic beliefs for adjusting instruction to learners' preconditions. Findings based on the new instrument indicate that apprentices show partly unfavourable epistemic beliefs. As a result, supporting learners in developing preferable beliefs provides an opportunity to increase their learning success.