Web sites are becoming a common way of communication, even for museums. If, at the beginning, web sites had very simple structures, now they are becoming more and more ambitious, comprehensive and, therefore, complex. For this reason usability becomes important, and its evaluation can not be neglected. This paper presents a systematic approach to usability evaluation of museum web sites, which specializes a more general methodology, named SUE. The heart of the methodology is a combination of inspection (where experts evaluators examine the application), and empirical testing (where a sample of end-user experimentally use the application). Characteristics also include the use of a model (to describe the application), of abstract tasks (to guide the inspection activities), and of concrete tasks (to guide the empirical testing). In this paper a brief description of the overall approach, a small sample of abstract tasks, and a few examples of inspection results are presented.