Investments in complex information systems by organizations reached a record high of U.S.$26.7 billion in 2004. Yet, organizations seldom use these systems to the fullest extent and attain the expected return on investment. This paper addresses the issue of system underutilization by investigating Extended Use, which refers to using more system features to support one's tasks. Extended Use was examined in the nomological networks of the IS Continuance (ISC) Model and Technology Acceptance Model (TAM). A field survey was conducted in a large manufacturing firm that had successfully implemented a popular enterprise resource planning solution for more than 2 years. All paths in both ISC and TAM were statistically significant. A synthesized model was later proposed and examined in a post hoc analysis. The results indicate that the synthesized model, as compared to ISC and TAM, explained slightly higher variances in Extended Use, Perceived Usefulness (PU), and Satisfaction. Specifically, both Perceived Ease of Use (PEOU) and PU both affected Extended Use. Interestingly, Satisfaction has no direct impact on Extended Use in the presence of PU and PEOU. In contrast to most technology acceptance research, PEOU has a stronger behavioral impact than that of PU. This research provides a framework that explains Extended Use and is one of the few studies that investigates IS use behavior that exceeds simple, shallow, and routine use.