Intermediate and mechanism of electro-oxidation of hydrogen (HOR) on Pt electrode was investigated in sulfuric acid solution under hydrogen atmosphere by in Situ infrared spectroscopic method utilizing the ATR-SEIRAS (Surface Enhanced Infrared Absorption Spectroscopy) mode. Adsorbed hydrogen species giving rise to infrared absorption band around 2090 cm(-1) appears at ca. 0.1 V vs. RHE, developing its intensity at lower potentials. Dependence of its band intensity on overvoltage and H-2 partial pressure was analyzed based oil the Volmer-Tafel mechanism of HOR with the rate determining dissociation of H-2. Frumkin isotherm with a positive interaction parameter reproduced the observed dependence reasonably well assuming proportionality between coverage and band intensity of the adsorbed hydrogen. Further, the band intensity was found to be independent of sulfuric acid electrolyte concentration for a given overvoltage and hydrogen partial pressure, in agreement with the above mechanism. These results showed that the adsorbed hydrogen species detected is the intermediate of hydrogen evolution and oxidation oil Pt, which is called overpotentially deposited hydrogen, H-OPD. (c) 2005 Elsevier B.V. All rights reserved.