The phenomenon that more knowledgeable learners do not benefit or even suffer from physically integrated text-picture formats, which work successfully with less knowledgeable learners, is called the expertise reversal effect in cognitive load theory (CLT) literature. A possible explanation of the expertise reversal effect is offered by CLT researchers, who argue that more knowledgeable learners suffer from extraneous cognitive load. Another explanation is given in the context of the cognitive theory of multimedia learning (CTML). In the CTML it is assumed that more knowledgeable learners can deal with difficult formats because of being able to invest germane load, that is applying relevant learning strategies. This study examined the different assumptions about the mechanism underlying the expertise reversal effect. Sixty students were assigned to either a group with low or high prior knowledge and to a group with either separated or integrated format, resulting in a 2 x 2 design. All students were eye tracked during learning. Subjective ratings of cognitive load could not support the extraneous load explanation. The results of the eye tracking data rather supported the assumptions made in the CTML.