The adsorption of atomic hydrogen on a platinum electrode in 0.1 M HClO4 electrolyte was studied in situ with Pt L-2.3 X-ray absorption spectroscopy (XAS). The Pt electrode was formed of highly dispersed 1.5-2.0-nm particles supported on carbon, and the Pt X-ray absorption fine structure data were collected at room temperature in transmission mode. A novel difference procedure utilizing the L-3 spectra at different applied voltages was used to isolate the effects of H adsorption on the Pt electrode in the XAS spectra. The results obtained in this work are compared with similar results recently reported for H/Pt in the gas phase and with results obtained from real-space full-multiple-scattering calculations utilizing the FEFF8 code on model clusters. The data reveal a number of important points that are similar to those found in the gas phase: (1) at low coverage the H is highly mobile and possibly delocalized on the surface, (2) at higher coverage it localizes into fcc sites, and (3) at very high coverage H is also found in atop sites presumably at or near edges. Point 3 is consistent with that found from previously reported spectroscopic data for Pt/C electrodes. Finally, a detailed interpretation of previously reported electrochemically modulated IR spectra, supporting the XAS analysis, suggests that the delocalized H exists at low coverage and has a profound affect on the water double layer.