Designing effective adaptive e-learning systems, from a usability perspective, represents a challenge because of the complexity of adaptivity in order to meet the diverse requirements of learners. Furthermore, there is a lack of well-designed experimental evaluation of adaptive e-learning systems in general, and of their usability in particular. The aim of this paper is the presentation of an adaptive e-learning system based on learner knowledge and learning style, and of the results of an initial experimental evaluation of the usability of its two modes of operation. This involves comparing the usability of an adaptive version of the system with the usability of a non-adaptive version, in a learning environment with 75 participants. The experiment produced significant results; they indicate that an adaptive e-learning system based on learner knowledge and learning style has a higher level of perceived usability than a non-adaptive e-learning system. This may also increase the level of satisfaction, engagement and motivation of learners and therefore enhance their learning.