S-(4-Benzoyl)phenylthiobenzoate (BpSBz) and 4-benzoyl-phenyl benzoate (BpOBz) in acetonitrile were photolyzed under steady-state and laser-flash conditions. Analysis of transients absorptions and final products from the photolysis of BpSBz showed that 4-benzoylphenylthiyl and 4-benzoyl radicals were formed with an initial quantum yield of 0.45. Benzaldehyde was the main final product. In contrast, photolysis of BpOBz did not yield benzaldehyde, and nanosecond laser-flash photolysis of BpOBz showed only a long-lived triplet state. These results suggest that CO bond cleavage does not occur as a primary photochemical reaction of BpOBz. The implications for the use of BpSBz as a photoinitiator of polymerization are discussed.