An exploratory study was conducted with users of a command-based system, MS-DOS, and the Graphical User Interfaces (GUI) of the Apple Macintosh. It was hypothesized that command-based interfaces and GUIs differ in the ease with which they afford attainment of particular concepts, and that in general the concepts investigated would be more readily attained by Macintosh users. The study attempted to assay subjects' knowledge of particular computer-related concepts, their ability to perform related operations, and the organization of that knowledge. Contrary to some theory (e.g., Tennyson and Cocchiarelli, 1986), a double dissociation between verbalizable conceptual knowledge and performance was observed. Results of the study suggested differences in the support provided by DOS and the Macintosh interface for development of knowledge of particular concepts and procedures, underscoring the potential value of understanding the detailed effects of particular interfaces, and classes of interface, upon user knowledge.