Abstract The reactivity of the triosmium clusters [Os 3 (CO) 10 (μ-dppm)] [dppm = bis(diphenylphosphino)methane] and [HOs 3 (CO) 8 {μ 3 -Ph 2 PCH 2 P(Ph)C 6 H 4 }] with tributylgermanium hydride (Bu 3 GeH) has been investigated. Oxidative addition of Bu 3 GeH to [Os 3 (CO) 10 (μ-dppm)] at 110 °C affords [HOs 3 (CO) 9 (GeBu 3 )(μ-dppm)] ( 1 ) which undergoes further decarbonylation upon prolonged heating to yield [H 2 Os 3 (CO) 7 (GeBu 3 ){μ 3 -Ph 2 PCH 2 P(Ph)C 6 H 4 }] ( 2 ) and [HOs 3 (CO) 8 (GeBu 3 )(μ-GeBu 2 )(μ-dppm)] ( 3 ). Control experiments confirm the bimodal formation of 2 and 3 from 1 during thermolysis. Cluster 2 reacts with additional Bu 3 GeH at 140 °C to give [Os 3 (CO) 6 (μ-GeBu 2 ) 2 {μ 3 -PhP(C 6 H 4 )CH 2 P(Ph)C 6 H 4 }] ( 4 ), a thermolysis product also produced from 3 . In contrast, oxidative addition of Bu 3 GeH to [HOs 3 (CO) 8 {μ 3 -Ph 2 PCH 2 P(Ph)C 6 H 4 }] occurs at room temperature to afford [H 2 Os 3 (CO) 8 (GeBu 3 ){μ 3 -Ph 2 PCH 2 P(Ph)C 6 H 4 }] ( 5 ), which decarbonylates at 110 °C to yield 4 . All new clusters have been characterized by elemental analyses and spectroscopic methods, and the molecular structures established by X-ray crystallography. Electronic structure calculations on the different hydride isomers based on 1 and 5 are reported.