Ultraviolet irradiation of H3Re3(CO)i2in degassed solution leads to quantitative formation of H2Re2(CO)8. Quantum yields measured at 313 and 366 nm are of the order of 0.1. Photolysis in the presence of CO also gives fragmentation and formation of HRe(CO)5and H2Re2(CO)5, but the latter thermally reacts with CO to form HRe(CO)5 and Re2(CO)10. The quantum yield of disappearance of H3Re3(CO)12in the presence of CO is 0.02. Photolysis of the analogous H3Mn3(CO)12cluster produces HMn(CO)5, Mn2(CO)10, and an unidentified brown precipitate. Experiments directed toward elucidating the mechanism of these photoreactions are discussed.