Many perspectives of user acceptance of Web sites have been examined, yet information systems research often overlooks the human-computer interaction aspects, particularly in the area of Web site usability. Web site usability has recently gained greater acceptance in information literature through the development of instruments by Palmer (2002) and Agarwal and Venkatesh (2002). This study conducted a confirmatory factor analysis of both instruments in an attempt to validate the two instruments. Our results found that the Palmer instrument exhibited satisfactory measurement properties, although allowing room for further refinement. The Agarwal and Venkatesh instrument, although useful as a practical metric, displayed poor validity for the underlying constructs that compose Web site usability. Validation of these instruments furthers their scope and potential use by researchers and practitioners in helping them better understand the capabilities of their Web sites, while providing a foundation for further refinement of the Web site usability construct.