Hydrogen chemisorption on small silica-supported Pt clusters was investigated using LI situ extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy, The clusters were found to exhibit a bulklike Pt first nearest neighbor (NN) distance (2.76 Angstrom) and low disorder while covered by chemisorbed hydrogen, In contrast, bare Pt clusters produced by heating in vacuo at 300 degrees C are characterized by a contracted Pt NN distance (2.66 Angstrom) and greater disorder. These effects are reversed by re-exposure of the bare Pt clusters to H-2 at 25 degrees C. The metal-support interface is characterized by a short Pt-O distance, irrespective of the presence of chemisorbed hydrogen. An apparent L-3 edge shift of 0.8 eV relative to bulk Pt is observed for the hydrogen-covered clusters. This shift is attributed to a decrease in the Pt L-3 edge resonance (white line) intensity, as no corresponding shift is observed at the L-2 edge, A hydrogen-related L-2.3 XANES feature at 9 eV appears with nearly equal intensity at each edge, This peak is assigned to electronic transitions from Pt 2p levels to H 1s-Pt 5d antibonding states with mixed d(3/2)-d(5/2) character. From the L-2.3 XANES analysis, we find that the number of unoccupied d states in hydrogen-covered Pt clusters is 23% less than in bulk Pt. In contrast, the L-2.3 XANES spectra of bare silica-supported Pt clusters are closely similar to those of bulk Pt : quantitative analysis reveals only a slight (4%) decrease in the number of unoccupied d states.