In situ x-ray absorption spectroscopy (XAS) in 1 M HClO4 was used to examine the electronic and structural effects of hydrogen adsorption on carbon supported Pt (Pt/C) and Pt alloyed with first row transition metals (Cr, Mn, Fe, Co, and Ni). In the case of Pt/C, potential excursions from the double layer region (0.54 V vs. RHE) to 0.0 V caused significant changes in the XAS spectra whereas none was observed for the alloys. The L(3) and L(2) x-ray absorption near edge structure indicated the generation of empty electronic states in the vicinity of the Fermi level due to adsorption of hydrogen, and the L(3) extended x-ray absorption fine structure indicated an increase in the coordination number of the first Pt-Pt shell from 9 to 11. The latter was attributed to a reversible surface restructuring process. Alloying of the Pt suppresses both the electronic and structural effects at 0.0 V. A comparison of the electrochemical kinetics for hydrogen oxidation by these electrocatalysts in a proton exchange membrane fuel cell indicated that alloying of the Pt had insignificant effects on the kinetics.