Ru supported on activated carbon, Al2O3, and MgO was assessed for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL). Role of H2O on the hydrogenation activity of Ru was studied by probe-adsorbed diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy. Ru supported on activated carbon showed a maximum productivity of 1.18 kgGVL kgcatalyst -1 h-1 with an insignificant loss in the activity after 72 h of continuous operation in the presence of H2O. Using pure LA, GVL rate was decreased by an order of magnitude (0.12 kgGVL kgcatalyst -1 h-1) within 6 h of reaction time. The physicochemical characteristics of the catalysts were examined by temperature-programmed desorption of NH3, CO pulse chemisorption, H2-temperature-programmed reduction, and X-ray photoelectron spectroscopy techniques. H2O-adsorbed DRIFT spectroscopic data revealed the reversible generation of surface -OH groups when aqueous LA was used as the substrate; consequently, Ru/C catalyst stability was also improved. Finally, on the basis of the kinetic and in situ spectroscopic data, a plausible surface-reaction mechanism is proposed for the vapor-phase LA hydrogenation to GVL in the presence of H2O over the carbon-supported Ru catalyst.