The Internet and World Wide Web have become an important medium of software applications. Web-based information systems are involved in many areas of our everyday life. Today quality issues concerning these systems are attracting growing attention. This paper presents a framework for modelling the quality of web-based information systems. First we use information flow analysis to break down a complex system into sub-systems. Then within each sub-system, we apply hazard analysis to find out the system failure mode, possible causes, and further, we uncover the relevant quality attributes. Using such methods, we develop a quality of Business-to-Customer e-business systems as an example. Three users, all having a rich experience in using e-commerce systems are interviewed in hazard analysis.