Most of the time, the reactions with reactants extracted from biomass are conducted in water to efficiently solubilise reaction intermediates and products. We will show here that water can also play an essential role in the catalytic activity, focusing on the conversion of levulinic acid (LA) into γ-valerolactone (GVL). LA can be obtained via cellulose hydrolysis and dehydration of such obtained glucose. GVL is an attractive platform molecule that can be derived from biomass and can be converted to a variety of chemicals, including biofuel additives. While Ru is a poor hydrogenation catalyst compared to Pt or Pd in gas phase, it is efficient in aqueous phase conditions to hydrogenate ketones such as the conversion of levulinic acid into GVL. Comparing experiments in THF and water, we showed that the activity of Ru/TiO2 catalyst is highly sensitive to its environment while Pt/TiO2 is not. Better insight is provided by periodic DFT calculations at the GGA level. The presence of a H-bonded water molecule can affect the adsorption of the reactant, intermediate or product. For instance, the obtained alcohol is not directly chemisorbed on the surface: it is strongly h-bonded to the chemisorbed water molecule. This water molecule dramatically reduces the energetic of the reaction pathway on Ru, hence enhancing the catalytic activity. Conversely, the presence of such a H-bonded water molecule doesn’t not affect the energetic of the reaction pathway on Pt. Finally, we predict also that this activation can be generalized to other oxophilic metals such as Co or Ni. This study demonstrates that the knowledge accumulated over the years for gas phase reactions at metallic surfaces is not systematically transferable to aqueous phase conditions.