Photoreactions of (CO)5ReMn(CO)3(alpha-diimine) complexes have been studied at different temperatures and pressures and by time-resolved UV-vis absorption spectroscopy on pico-to millisecond time scales. Irradiation in to the MLCT absorption band in CH2Cl2 or in toluene/CCl4 Mixtures produces Re(CO)5Cl and Mn(CO)3(alpha-diimine)Cl whereas the substituted product (CO)5ReMn(CO)2(PPh3)(alpha-diimine) is formed in the presence of PPh3. Irradiation with 9,10-phenanthrenequinone leads to semiquinone radical complexes. Similar quantum yield values were obtained for all these reactions and also for various photoreactions taking place in mixtures of 9,10-phenanthrenequinone and PPh3. These observations indicate that all the final products are formed from the same primary photoproduct that was characterized spectroscopically in low-temperature solutions as a CO-(semi)bridged (CO)4Re(mu-CO)Mn(CO)2(alpha-diimine) species formed by CO photodissociation from the parent complex. Very similar apparent volumes of activation found for the reaction of (CO)5ReMn(CO)3(4,4’-dimethyl-2,2’-bipyridine) with CH2Cl2 (17.2 +/- 1.3 cm3 mol-1) and PPh3 (15.7 +/- 0.5 cm3 mol-1) are again consistent with the involvement of an identical primary photoprocess in both reactions. Moreover, the apparent activation volumes are similar to those of thermal CO-loss reactions in analogous complexes. Finally, picosecond (0 ps-10 ns) and nanosecond (20 ns-200 mus) laser flash photolysis suggested that the formation Of (CO)4Re(mu-CO)Mn(CO)2(alpha-diimine) is the only primary photoprocess, the usual Re-Mn bond homolysis being absent. Picosecond data indicate that it is a very rapid, most probably subpicosecond, process.