By means of pulse radiolysis, the one-electron reduction potentials of 4-substituted benzoylthiolates E degrees(4-XPhC(O)S-./4-XPhC(O)S-), where X is CH3, CH3O, CF3, and CN, were measured in aqueous solutions. The kinetics of beta-fragmentation reactions of the 4-XPhC(O)S-. radicals to form the corresponding 4-XPh. radicals and COS (i.e., 4-XPhC(O)S-. --> 4-XPh. + COS) were also determined. The pK(a)s of the corresponding acids (4-XPhC(O)SH) were measured by a spectrophotometric method. Thus, the values of E degrees(4-XPhC(O)S-., H+/ 4-XPhC(O)SH) and the S-H bond strength of the 4-XPhC(O)S-H were calculated. The substituent effects on the redox potential, the pK(a), and the kinetics of their beta-fragmentation reactions were examined.