The decomposition of formic acid catalyzed by a cationic hydridoplatinum tetradentate phosphane complex was studied by NMR spectroscopy, isotope labeling and DFT calculations. In the initial step dihydrogen is formed by the reaction of the formic acid with the hydrido ligand yielding a platinum formate cation. The formato ligand is then transformed to CO2 by a beta-H transfer of the formate C-H bond to the Pt center revealing the initial cationic Pt-H complex. The reaction sequence was illuminated by deuterium and C-13 isotope labeling revealing the cationic Pt-D complex as only Pt complex product if a deuterated formate ligand was used. The transfer of the hydrido ligand to an ensuing coordination site at the Pt center is assisted by the labile bonding character of the phosphane ligand. The activation energy was calculated to be +110 kJ/mol above the initial formate complex. The decomposition reaction was observed to be completely reversible yielding formic acid from CO2 and H-2 at a pressure of around 40 bar. Thus, this reaction provides an excellent example for dihydrogen storage and release on demand using a Pt complex as catalyst.