Surface area data, steady-state hydrocarbon conversion efficiency data, and hydrocarbon emissions results have been determined for catalysts collected by the U.S. Environmental Protection Agency from properly maintained 1981 and 1982 model year vehicles. Catalysts covered in this study were limited to those with three-way-plus-oxidation monolith technologies. Catalyst surface areas were measured using the BET method, conversion efficiencies were measured on an exhaust gas generator, and emissions results were determined using the Urban Driving Schedule of the Federal Test Procedure.Results indicate that correlation of catalyst surface area data with hydrocarbon conversion efficiency data and hydrocarbon emissions results is significant for the sample studied. The correlation of catalyst surface area with hydrocarbon conversion efficiency indicates that a significant percentage of the variation in hydrocarbon conversion efficiency can be attributed to the variation in in-use catalyst surface area. Also, the correlation between catalyst surface area data and hydrocarbon emissions data indicates that a portion of the variation in Federal Test Procedure hydrocarbon emissions is due to the variation in in-use catalyst surface area.