For many companies, effective web presence is indispensable for their success to the global market. In recent years, several methods have been developed for measuring and improving the effectiveness of commercial sites. However, they mostly concentrate on web page design and on access analysis. In this study, we propose a methodology of assessing the quality of a web site in turning its users into customers. Our methodology is based on the discovery and comparison of navigation patterns of customers and non-customers. This comparison leads to rules on how the site’s topology should be improved. We further propose a technique for dynamically adapting the site according to those rules.