Ab initio molecular dynamics simulations of coadsorbed hydroxyl and water species on a Rh(1 1 1) surface have been performed using the Perdew-Wang 91 (PW91) density functional. The results reveal the possibility of an apparent diffusion of the OH on the surface through a Grotthus-like mechanism. The estimated proton exchange rate of around 3 ps(-1) is higher compared to that in bulk water. The surface-bonded hydroxyl forms a complex with two neighboring water molecules. The complex has hydrogen bond distances different from those between water molecules. (C) 2002 Elsevier Science B.V. All rights reserved.