To succeed in the highly competitive e-commerce environment, it is vital to understand what customers want in a website to enhance conversion and retention rates of websites. Numerous contingent usability attributes have been the result of past usability studies and there are no unified framework or model to classify these attributes. This study adopted the most prominent model of IS success to provide a parsimonious and unified view of usability. Past usability studies have also focused on measuring the outcomes of usability and neglected the relationship between the actual usability dimensions and the intention to purchase. When examining the relationship between usability and the intention of purchase, this study compares the impact of usability on conversion and retention. In the comparison between conversion and retention, the empirical results demonstrate that the system quality is more important for customer conversion whereas service quality is more important for customer retention.