To study the users and use of the subject searching capability offered by the new online public catalog at the New York State Library (NYSL) a study was begun before the catalog change in mid-1983 and continued for a considerable time after the change. Data were collected during three 1-week periods; one preceded the catalog change and two followed. It was thus possible to detect differences associated with the catalog change, and also to determine whether any of the differences detected were likely to be permanent or transient. The methodology of the study was adapted from earlier use studies of NYSL. As in the earlier studies, library user traffic was measured unobtrusively throughout the study weeks, and frequent short, standardized interviews were conducted with a sample of users. In this research, the method was applied simultaneously to the population of users of the public catalog, by unobtrusively counting and timing users of the online computer terminals and microfiche viewers associated with the public catalog, and by interviewing a sample of catalog users as they were leaving the catalog-access devices. Factors considered for possible impact effects included: the amount and distribution of use of the public catalog; the proportion of subject searches; the duration of catalog searches; their success rate; the use of librarian assistance with catalog searches; user preference for online catalog access versus microfiche catalog access; the motivation of catalog searches; and the affiliations of catalog users. References, the User Exit Interview Guide, and the Catalog User Interview Guide are appended. (THC)