Complex OsH2(CO)(PiPr3)2, generated from OsH2(CO)(η2-CH2═CHEt)(PiPr3)2 by dissociation of the olefin, promotes the release of 1 equiv of H2 from ammonia-borane and the formation of polyaminoborane. The dehydrogenation rate law is of the form d[H2]/dt = k[Os] with activation parameters of ΔH‡ = 19.6 ± 2.5 kcal·mol–1 and ΔS‡ = 6 ± 9 cal·mol–1·K–1. On the basis of kinetic results, spectroscopic NMR observations, and DFT calculations, the dihydride-dihydrogen derivative OsH2(η2-H2)(CO)(PiPr3)2 is proposed as a key intermediate for the dehydrogenation. Its formation involves the concerted BH- and NH-hydrogen transfers from the amine-borane to the metal center and to a hydride ligand, respectively.