We have analyzed the surface oxidation process of Pt nanoparticles that were uniformly dispersed on a glassy carbon electrode (Pt/GC), which was adopted as a model of a practical Pt/C catalyst for fuel cells, in NZ purged 0.1 M HF solution by using angle-resolved, grazing-incidence X-ray photoelectron spectroscopy combined with an electrochemical cell (EC-ARG1XPS). Positive shifts in the binding energies of Pt 4f spectra were clearly observed for the surface oxidation of Pt nanoparticles at potentials E > 0.7 V vs RHE, followed by a bulk oxidation of Pt to form Pt(II) at E > 1.1 V. Three types of oxygen species (H2Oad, OHad, and O-ad) were identified in the 0 Is spectra. It was found for the first time that the surface oxidation process of the Pt/GC electrode at E < ca. 0.8 V (OHad formation) is similar to that of a Pt(111) single-crystal electrode, whereas that in the high potential region (O-ad formation) resembles that of a Pt(110) surface or polycrystalline Pt film.