The mechanism of reversible hydrogen evolution and oxidation reactions (HER and HOR) on Pt electrode is explained and a thermodynamic contradiction related to participation of an adsorbed hydrogen atom as intermediate in these processes is resolved. Modeling invoking a newly proposed intermediate - the molecular hydrogen ion - describes experimental HER and HOR results quantitatively. The findings based on thermodynamic balance and experimental data provide further insights into understanding and interpretation of hydrogen-related surface processes relevant to water splitting, hydrogen generation, and storage. (C) 2013 Published by Elsevier B.V.