We characterize usage and problems for Web applications, evaluate their reliability, and examine the potential for reliability improvement. Based on the characteristics of Web applications and the overall Web environment, we classify Web problems and focus on the subset of source content problems. Using information about Web accesses, we derive various measurements that can characterize Web site workload at different levels of granularity and from different perspectives. These workload measurements, together with failure information extracted from recorded errors, are used to evaluate the operational reliability for source contents at a given Web site and the potential for reliability improvement. We applied this approach to the Web sites www.seas.smu.edu and www.kde.org. The results demonstrated the viability and effectiveness of our approach.