The roles of adsorbed hydroxyl radicals, OH, at a high temperature and adsorbed hydrogen atoms, H, in an acidic solution were investigated in the electrochemical reactions on Pt electrode by using potentiodynamic polarisation experiment, cyclic voltammetry and constant-potential electrolysis combined with UV/VIS analysis. From the analysis of the polarisation curves obtained from Pt electrode in a 0.185 M H3BO3 solution at 473 K, it was found that the reducing capability of dissolved hydrogen is significantly enhanced due to the increases of the mass transfer and the electron transfer rates. Especially, it is suggested that the stable Pt-OHad plays a significant role in the passivation reaction in the potential range from 0.60 to 0.75 V-SHE. From the analyses of the experimental results for the electrochemical reduction of UO22+ ions on Pt surface in a 1.0 M HClO4 solution, it is recognised that the reduction reaction of UO22+, to U4+ 2 ions is strongly dependent on the hydrogen atoms adsorbed on Pt electrode (indirect reduction of UO22+ ) as well as on the electrons transferred from Pt of UO22+). In addition, the of UO22+ reduction mechanism UO22+ ions involved in Pt-H-ad is also proposed.