The fluorescent molybdenum(V) oxo complex MoOCl 4 (H 2 O) - , and oxomolybdenum(V) phosphine complexes derived from it, undergo thermal and photochemical reactions in acetonitrile with the series of phosphines PEt 3 , PEt 2 Ph, PEtPh 2 , and PPh 3 . PEt 3 reacts spontaneously to form several products: crystalline (PPh 4 )[Mo I I I Cl 4 (PEt 3 ) 2 ] and (PPh 4 ) 2 [Mo V OCl 5 ].2CH 2 Cl 2 ; and a maroon oil that contains OPEt 3 , Mo I V OCl 2 (PEt 3 ) 3 and Mo I V Cl 4 (PEt 3 ) 2 . The Mo(III) and OPEt 3 products show that oxygen atom transfer has occurred. PEt 2 Ph and PEtPh 2 show no significant redox activity with oxomolybdenum(V) in the dark, but irradiation (λ>320 nm, CH 3 CN solution) leads to reduction of Mo(V) and production of the corresponding phosphine oxides. The molybdenum(III) product, cis-mer-MoCl 3 (OPEt 2 Ph) 2 (PEt 2 Ph), was isolated from the PEt 2 Ph experiment. PPh 3 is not oxidized under these conditions; instead, spectral evidence suggests that photoinduced ligand substitution occurs, producing MoOCl 3 (PPh 3 ) 2 . Selective irradiation of the longer-wavelength Mo(V) absorption bands leads to similar photoredox reactions, but much more slowly. X-ray analyses of two reaction products, (PPh 4 ) 2 [Mo V OCl 5 ].2CH 2 Cl 2 and cis-mer-Mo I I I Cl 3 (OPEt 2 Ph) 2 (PEt 2 Ph), are reported.