Adsorbed hydrogen and phenol on Pt nanoparticles during (electro)catalytic hydrogenation are explored by combining X-ray absorption spectroscopy and ab initio simulations. Direct evidence for two types of Pt-C bonds at the surface of the metal particles detected by X-ray absorption spectroscopy suggest strong bonding between metal and the carbon support as well as adsorption of phenol nearly parallel to the surface. Hydrogen and phenol compete for accessible Pt sites. The surface concentrations are compatible with the proposal that atomic hydrogen and chemisorbed phenol are the species reacting in the rate determining step of hydrogenation in the presence and absence of an external cathodic potential. During electrocatalytic hydrogenation the external electric potential controls the concentration of species on the surface, but does not impose structural or electronic property changes of the Pt compared to Pt particles in presence of hydrogen gas. Increasing reaction rates with increasing cathodic potential are attributed to the increase in chemical potential of adsorbed H. (C) 2018 Published by Elsevier Inc.