The number of Institutional Repositories (IRs), such as DalSpace, has been growing in the past few years. However, most IRs are not widely used by the intended end users. Evaluating the user interfaces is an essential part of any process to increase users' acceptance of IRs. There are two foci of this thesis: to evaluate the usability of DalSpace's interface using Nielsen's heuristics to uncover usability problems for development purposes and to examine the differences between user-interface experts and non-experts in uncovering problems with the interface. To apply the heuristics to the interface, I formed user profiles (also known as personas) to represent potential end-users groups. These profiles helped to communicate users' needs, abilities, tasks, and problems. To produce a reliable list of usability problems by applying the heuristic evaluation approach, I examined the impact of expertise on the quality of the results. From the individual heuristic analyses (by both experts and novices), I distilled 66 usability problems classified by severity. Also, the frequency of each violated heuristic was used to assign priority to the uncovered usability problems as well as the severity level. The results of applying the heuristic evaluation show that both experts and non-experts can uncover usability problems. In fact, the ability to find difficult and easy problems was recorded for both types of evaluators. However, experts tend to reveal more serious problems, while novices uncover less severe problems. Interestingly, the best evaluator (who found 21% of the total number of problems) was a novice. Our results indicate that we cannot rely on one evaluator even if the evaluator is an expert. The administrative interface is out of the scope of the thesis; however, the usability of the interface should be examined for improvement purposes. More user profiles should be formed to represent additional user groups for more interfaces including the end user and staff's interfaces. Both results from the user profiles and the list of usability problems will be given as tools to the development team for improvement.