This paper addresses the issue of usability testing in a component-based software engineering environment, specifically measuring the usability of different versions of a component in a more powerful manner than other, more holistic, usability methods. Three component-specific usability measures are presented: an objective performance measure, a perceived ease-of-use measure, and a satisfaction measure. The objective performance measure is derived from the message exchange between components recorded in a log file, whereas the other measures are obtained through a questionnaire. The power of the measures was studied in an experimental setting. Eight different prototypes of a mobile telephone were subjected to usability tests, in which 80 subjects participated. Analyses of the statistical power of these measures show that the component-specific performance measure can be more powerful than overall usability measures, which means fewer users are needed in a test.