In 1990, the Online Catalog Advisory Committee (OCAC) of the University of Illinois at Urbana-Champaign (UIUC) charged a subcommittee with developing and implementing ways to measure the impact of the expanded online catalog on staff, users, collections, and library operations. The UIUC's online public access catalog, ILLINET (Illinois Library and Information Network) Online, links the circulation systems of about 40 institutions in Illinois. The subcommittee reviewed literature in this area and developed a two-stage study, with baseline and comparative data at one year. Instruments to measure impact were developed and introduced to staff in orientations at 36 sites. Over 8,000 questions were asked of university patrons and external users of the library in each of the two years. Study results make it overwhelmingly clear that the new online services are very popular and are being used at an increasing rate. It is important that staff and users be aware of how to make effective use of these resources. Recommendations are made for improved training for staff and users, improved and enhanced data collection for evaluation, improved resource availability and enhanced hardware, and increased attention to the availability of periodicals. Appendixes contain the data collection instruments, a sample of the data collection code, and 11 statistical tables. (Contains 20 references.) (SLD)