Although near-UV irradiation of Rh(PMe(3))(2)(CO)Cl (1) has been shown to lead to efficient carbon-hydrogen bond activation, the title reaction is promoted with much greater efficiency by shorter wavelength (lambda < ca. 320 nm) irradiation : for example, the carbonylation quantum yield at 314 nm is found to be 1.5 x 10(-3) vs 2.0 x 10(-5) at 366 nm. A comprehensive mechanistic study reveals that the shorter wavelength irradiation is required for two separate steps in the catalytic cycle. Photoextrusion of CO, which occurs upon near-UV irradiation and leads to efficient C-H bond activation, is not a major step toward carbonylation under the reaction conditions of the present study nor, apparently, is any other dissociative photoreaction. It is proposed that the first reaction step is the oxidative addition of a benzene C-H bond to an intact photoexcited state of 1. Independently, Field and co-workers have generated and characterized the resulting six-coordinate phenyl hydride complexes, Rh(PMe(3))(2)(Ph)(CO)HCl (2) under low-temperature conditions. Kinetic studies of the catalytic reaction indicate the involvement of a photoactive intermediate, the decay rate of which is in good agreement with the activation parameters determined by Field et al. for the low-temperature decay of the phenyl hydride complexes, 2. Short wavelength light is required for the efficient photoreaction of 2, possibly because longer wavelength light is not significantly absorbed, particularly in competition with 1. The carbonylation efficiency increases with CO pressure; this is attributable to the reaction of CO with the unsaturated benzoyl complex, Rh(PMe(3))(2)(COPh)HCl, which results from CO insertion into the Rh-phenyl bond of 2. Reversible loss of benzoyl chloride from the CO addition product, Rh(PMe(3))(2)(COPh)(CO)HCl, is shown to lead to the formation of C6H5CDO and C6D5CHO when the reaction is conducted in C6H6/C6D6 mixtures.