A method of combining thin-layer chromatography with activation analysis to measure quantitatively the individual impurities in tricresyl phosphate (TCP) is described. The relative amounts and types of impurities found in commercial grades of TCP are discussed. Preparative thin-layer chromatography and iron powder chromatography are applied to produce TCP samples with known concentrations of several impurities. The use of the four-ball wear tester as a method of determining trends in wear values as a function of base stock and additive quality is demonstrated. The relationship of acid phosphate rather than TCP concentration to antiwear properties of compounded lubricants is suggested by transition load studies.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference held in Chicago, Illinois, October 17–19, 1967.