Prior knowledge and cognitive styles are considered important determinants in adaptive hypermedia learning systems (AHLSs) as they influence how learners select information to put into memory. However, there is a need to investigate how they influence learner performance and perceptions prior to comparing them and establishing if they can be used together to maximise learning in AHLSs. To this end, this study investigated the effects of prior knowledge on the use of AHLSs to set the foundation for the comparison. 60 students participated in this study. The results showed that, in general, adapting to prior knowledge improves learner performance and perceptions, especially for users with low prior knowledge. However, the results also indicated that the relative improvement in learner performance is significantly higher than that of perceptions when using the AHLS. The implications of the design of AHLSs are discussed by the paper.