We have studied the mechanism of reduction by Noyori's Ru(II) Arene Diamine hydrogenation catalyst. This species is a transfer hydrogenation catalyst, utilizing isopropanol as the terminal reductant. Based on a detailed kinetic study, including the determination of kinetic isotope effects, we have concluded that the mechanism of reduction includes concerted transfer of the hydride and proton to a polar unsaturated species; Noyori has termed this type of transfer "metal-ligand bifunctional catalysis."