Pt cluster size strongly influences the reactivity of surfaces for C-H bond activation during CH4 steam reforming. C-H bond activation turnover rates for H2O reforming of CH4 increased with decreasing Pt cluster size, suggesting that coordinatively unsaturated surface Pt atoms are more reactive than Pt atoms in low-index surfaces prevalent in larger clusters. Near-edge fine structure in X-ray absorption spectra clearly shows electronic changes in Pt clusters relative to bulk Pt crystallites, though differences in the 5d electronic density among samples with different cluster size could not be related directly to Pt surface atom coordination. These results suggest that catalytic consequences of cluster size arise from surface structure changes leading to coordinative unsaturation of surface atoms. X-ray absorption and small angle X-ray scattering techniques were used to measure the size of Pt metal nanoclusters and the results are compared with H-2 chemisorption and transmission electron microscopy data. (C) 2004 Elsevier B.V. All rights reserved.