A density-functional study of homolytic and heterolytic O-H bond cleavage in the dehydrogenation reaction of H2Oads has been carried out on ruthenium sites in bimetallic Pt(111)/Ru nanoclusters of varying surface morphology. The Ru sites are either produced by co-deposition, forming an alloy, or by sequential deposition. The reaction energies and activation barriers for H2Oads, dissociation on the Ru sites are estimated. On the basis of the energetics of H2Oads(Ru) dissociation on the Ru sites, the sequentially deposited Pt/Ru bimetallic cluster surface is predicted to be more catalytically active in water activation and Co-ads oxidation than the alloy surface. (c) 2005 Elsevier B.V. All rights reserved.