Eye-tracking equipment is used to assess how well a subject comprehends UML class diagrams. The results of a study are presented in which eye movements are captured in a non-obtrusive manner as users performed various comprehension tasks on UML class diagrams. The goal of the study is to identify specific characteristics of UML class diagrams, such as layout, color, and stereotype usage that are most effective for supporting a given task. Results indicate subjects have a variation in the eye movements (i.e., how the subjects navigate the diagram) depending on their UML expertise and software-design ability to solve the given task. Layouts with additional semantic information about the design were found to be most effective and the use of class stereotypes seems to play a substantial role in comprehension of these diagrams.