In the UV photolysis of M-2(CO)(10), M = Mn or Re, flash photochemical methods were used for the determination of the primary product, i.e., nonacarbonyl (M-2(CO)(9)) and pentacarbonyl (M(CO)(5)) quantum yields. The quantum yield based action spectra are very metal dependent and indicative of different mechanisms for the Re and Mn photochemistry. In the figure, the action spectra for the Re decacarbonyl photoproducts. phi(P) for Re(CO)(5) and phi(N) for Re-2(CO)(9), are superimposed with the absorption spectrum. The results in the photochemistry of Re-2(CO)(10) suggest that the upper electronic states, i.e., populated by irradiating below 340 nm, produce some Re(CO)(5).