In fuel cells, the formation of platinum oxides on the Pt surface interferes with the Oxygen Reduction Reaction (ORR). The chemical composition of Pt oxides formed on carbon-supported Pt nanoparticles has been postulated for the first time to be hydroxylated platinum PtOH and hydroxylated platinous PtOH+ species. The thermodynamics of electrocapillary was applied to determine the charge number for Pt oxidation in various pH under a constant ionic strength perchlorate solutions. The charge number of the Pt oxide increased with the holding potential from -0.9 up to -1.5 electrons/H+ at 0.765 and 0.89 V vs. SHE, respectively. This indicates the coexistence of multi oxides, where PtOH is initially formed and is further converted at higher potentials to PtOH+. The charge number for H adsorption was measured as expected to be +1, increasing the confidence in the technique. The charge number allows the calculation of individual oxide coverages, explaining the previously intriguing reports that the fraction coverage was larger than 1 under the simplified assumption of one-electron transfer per Pt atom and without the need to assume a place-exchange mechanism. It is concluded that under increasing holding potential the coverage of PtOH+ increases while that of PtOH decreases. The formation of both oxides can be periodically reversed by a short switch to lower potentials, E < 0.6 V vs. SHE. The reduction of the oxides and consequently the recovery of Pt activity are found to be relatively fast. (C) 2014 Elsevier B.V. All rights reserved.