Usability testing on library search tools was conducted with ten students and eighteen library staff members at Syracuse University. The study addressed three research questions: (1) Do the ways in which librarians carry out search tasks on the library Web site vary from those of student users?; (2) Are those variations indicative of different mental models, i.e. different experience with and knowledge about the content and search tools on the Web site?; and (3) If there are differences, what are the implications of those different models for the usability of the site by students? Participants were tasked by using the library's Web site to locate both known items and items about a subject in the library's collection. The two groups differed in the success with which they were able to complete the tasks, as well as the time used. There were differences in the selection of search tools, the syntax used within the search box, the expectations of how library resources are organized, and the level of persistence as tasks were performed. Students' behavior is characteristic of different mental models for how search tools work. The results are applicable to the design of the library's Web site and have implications for how library staff conduct reference and instruction services. (Contains 5 figures.)