The CO substitution reaction of Re2(CO)l0 by triphenylphosphine was investigated photochemically under vacuum. In contrast to the previous studies, three sterically crowded, triply substituted dirhenium complexes, i.e. 1-axial-2',4'-diequa-torial-Re2 (CO)7 (PPh3)3 1-axial-2,2'-diequatorial-Re2 (CO)7 (PPh3 )3 and 2,2'4'-tri-equatorial-Re2(CO)7(PPh3)3 were isolated and characterized. A fourth compound,which was tentatively assigned as Re2(CO)6(PPh3)4, was also isolated. Contrary to the Mn system, the [Re(CO)n(PPh3)5-n] radical, which is believed to be the intermediate of the photochemical substitution reaction, could not be detected by EPR spectroscopy. The subtle differences in substitution chemistry between rhenium carbonyl and manganese carbonyl complexes are the manifestation of the delicate balance of metal-metal bond strength and the steric repulsion introduced by the bulky phosphorus-containing ligands.