The shift from petroleum to biomass feedstock triggers the search for efficient catalysts working in aqueous environment. Water is necessary to solubilise reactants and products. However, it can also strongly modify the kinetics and the selectivity of a reaction compared with gas phase conditions. Thus, we need a reliable and simple strategy to probe this effect. Based on the well-established periodic slab model, we proposed to use a micro-solvation approach within the DFT framework. At a minimal cost, we showed recently that a single water molecule can strongly affect the de-hydrogenation of alcohols on Rh(111) 1,3 and Pt(111).2,3 In presence of a chemisorbed water molecule, an alcohol is preferentially H-bonded to this water molecule, its oxygen being far from the metallic surface. This co-adsorption process is synergetic leading to stronger adsorption energies. Despite the unusual distance, the OH bond scission is strongly activated on Rh, much less Pt. On the other hand, the CH scission is barely affected. We will show that this water assistance is even stronger for the reverse reaction, the catalytic hydrogenation of ketones. Oxophilic metals (Ru, Rh, etc.) are more active with a drop of the energy span of ~0.40 eV while less oxophilic metals such as Pd or Pt are barely affected. Thus, in presence of water, platinum is not anymore the metal of choice to reduce ketones into alcohols and should be replaced by iridium or ruthenium for instance. This water assistance effect may explain why Ruthenium is widely used in biomass conversion. References 1C. Michel, F.Auneau, F.Delbecq, Ph. Sautet, ACS Catalysis, 2011, 1, 1430. 2.S. Chibani, C. Michel, F. Delbecq, C. Pinel, M. Besson, Catalysis Science & Technology, 2013, 3, 339 3. D. Loffreda, C. Michel, F. Delbecq, Ph. Sautet, Journal of Catalysis, 2013, 308, 374