The deposition of platinum on titanium at open-circuit potential from an aqueous 0.1 M HClO4 solution containing K2PtCl6 was studied. On freshly polished titanium, spontaneous deposition of platinum takes place through a displacement reaction between Ti(O) and dissolved Pt(TV). At very short times of deposition, fine dispersions of platinum crystals were formed on titanium. At longer deposition times, the size of the platinum crystals increases whereas their number decreases due to coalescence and agglomeration processes. On titanium covered by a thin oxide film, the rate of deposition is very low and photocatalytic deposition of Pt on the TiO2 surface occurs simultaneously with the displacement reaction. The hydrogen evolution reaction (her) was used as a probe reaction for testing both the catalytic activity and deposition conditions. There is evidence that the catalytic activity of the Pt crystals for the her increases with decreasing crystal size.