Abstract Various adlayers were grown on the (111) surface of a platinum crystal by four different oxidizing treatments. The resulting surface phases were characterized by X-ray photoelectron spectroscopy. According to the Pt 4f and O 1s XPS spectra, treatment in 0.1 MPa O 2 at 900 K yielded about two monolayers of PtO 2; anodic polarization in 0.5M H 2SO 4 acid electrolyte at 3 V versus standard hydrogen electrode gave a thick (> 5 nm) layer of Pt(OH) 4, while by polarization in 1M NaOH base electrolyte at 3 V versus Ag/AgCl reference electrode a thick film of an oxyhydroxide (approximately PtO(OH) 2) was formed. Etching in boiling conc. HNO 3 led to a thin layer of about 1 nm of a hydrated oxide, PtO 2 · xH 2O. Identification of the different surface phases was supported by comparative experiments with bulk PtO 2. The thermal stability of all compounds was investigated by heating the samples in ultrahigh vacuum. They all decomposed at about 400 K. Initially, not a pure metal phase was formed, but a mixed phase containing Pt metal and oxide, which was stable over a wide temperature range. No oxygen could be detected on the surface at 1070 K by XPS or Auger spectroscopy.