In heterogeneous catalysis, water is of great importance as a solvent (biomass valorization, etc.) but also as a reactant or product (water gas shift reaction, etc.). Its presence can strongly modify the behavior of metal catalysts, assisting or inhibiting reactions. Under oxidative conditions, water is also expected to generate hydroxyl groups on such metallic surfaces. Using DFT periodic calculations, we will analyze the effect of those co-adsorbates on the C-H and O-H scissions in water and ethanol. Water preferentially adsorbs as a H-bonded dimer than as two monomers: the H-bond donor is chemisorbed and the acceptor is farther from the surface. In this water dimer, the OH scission is facilitated compared with the isolated water for various metals and various DFT functionals. This promotion effect is much stronger on oxophilic metals such as Ru or Rh than for later metals such as Pt.[1] In presence of chemisorbed water, the adsorption of ethanol is analogous to the water dimer, the ethanol replacing the physisorbed H-bonded water molecule. Here again, the OH scission is strongly promoted on the oxophilic Rh and weakly on Pt, a later metal. However, the CH scission is inhibited, limiting the overall dehydrogenation process on Pt.[2,3,4] Though, under oxidative conditions, water is a source of hydroxyls that drastically modify the activity of Rh and Pt catalysts: the presence of co-adsorbed hydroxyl facilitates the alcohol dehydrogenation, with an almost barrierless OH scission and a low barrier CH scission.[3,4] [1] C. Michel, F. Göltl, Ph. Sautet Physical Chemistry Chemical Physics, 14 (2012) 15286 [2] C. Michel, F. Auneau, F. Delbecq, Ph. Sautet, ACS Catalysis, 1, (2011) 1430 [3] S. Chibani, C. Michel, F. Delbecq, C. Pinel, M. Besson, Catalysis Science and Technology, 3, (2013) 339 [4] D. Loffreda, C. Michel, F. Delbecq, Ph. Sautet, Journal of Catalysis (2013) 10.1016/j.jcat.2013.08.011