The quality of web information systems is closely related with their users' expectations. In order to create good quality systems, web designers should understand how users perceive service quality of various performance attributes such as security, usability and information quality, and which ones they value the most. In this study, users' service quality perceptions of two different web sites were investigated. Data were obtained from 1,900 users of web sites of a for-profit online bookstore and a not-for-profit national information center through the E-Qual Index that was administered online. Users were asked to rate to what extent each web site satisfied the 22 performance attributes included in the E-Qual Index along with the importance they attached thereto. Quadrant analysis and gap analysis were used to analyze the data to find out the strengths and weaknesses of the web sites investigated. Moreover, performance attributes that need to be improved or that were "undervalued" by the users were identified. Findings indicate that each web site satisfied more than half the performance attributes listed in the E-Qual Index. Users attached more importance to performance attributes such as ease of use, provision of timely and relevant information and safety. Findings can be used to remedy the shortcomings of each web site and improve the web service quality.