The paper proposes a contingent, learner-centred usability evaluation method and a prototype tool of such systems. This is a new usability evaluation method for web-based learning systems using a set of empirically-supported usability factors and can be done effectively with limited resources. During the evaluation process, the method allows for the prioritisation and selection of the most relevant usability factors for each web-based learning system. During the reporting process, the method allows for the prioritisation of usability problems to be dealt with in system improvement. The method is then implemented in an online prototype of a tool that semi-automates the evaluation process. (Contains 17 figures and 3 tables.)