Monoorganophosphines react with Os 3(CO) 12 in presence of trimethylamine oxide to give the phosphine-substituted cluster [Os 3(CO) 11(PRH 2)] and the phosphido-bridged hydrido cluster [(μ 2-H)Os 3(CO) 10(μ 2-PRH)] (R = C 6H 5, p-CH 3OC 6H 4, C 6H 11). The thermolysis of [Os 3(CO) 11(PRH) 2)] gave, under different conditions, [(μ 2-H)Os 3(CO) 10(μ 2-PRH)] and [(μ 2-H) 2Os 3(CO) 9(μ 3-PR)], and the thermolysis of [(μ 2-H)Os 3(CO) 10(μ 2-PRH)] gave [(μ 2-H) 2Os 3(CO) 9(μ 3-PR)] (R = C 6H 5, C 6H 11). All the compounds have been studied by IR, NMR ( 31P and 1H) and mass spectrometry. The structures of [(μ 2-H)Os 3(CO) 10(μ 2-P(C 6H 5)H)] and [(μ 2-H) 2Os 3(CO) 9(μ 3-PC 6H 5)] have been determined by X-ray crystallography. The results obtained for the system Os 3(CO) 12/PRH 2 demonstrate a stepwise transformation of a terminally bonded PRH 2 ligand to a doubly bridging μ 2-PRH unit and finally to a triply bridging μ 3-PR group by successive hydrogen transfer and CO substitution processes.