Several previous studies have relied on religious affiliation and proximity to Catholic schools as exogenous sources of variation for identifying the effect of Catholic schooling on a wide variety of outcomes. Using three separate approaches, this study examined the validity of these instrumental variables. Evidence was obtained on the validity of using three sources of variation in Catholic school attendance: religious affiliation, proximity to Catholic schools, and the interaction between religion and proximity. The main data set for these analyses was the National Educational Longitudinal Survey of 1988 (NELS:88), which provided data on 24,599 eighth graders in its first data collection, but results are also presented based on the National Longitudinal Study of the High School Class of 1972 (NLS-72), which contained data on 22,652 high school seniors in 1971-1972. Findings show that none of the candidate approaches is a useful source of identification of the Catholic school effect, at least in currently available data sets. An appendix compares bivariate probits and two state least squares statistical approaches. (Contains 6 tables and 33 references.) (Author/SLD)