The mechanism of platinum surface electro-oxidation is examined by combined cyclic-voltammetry (CV), in situ electrochemical quartz-crystal nanobalance (EQCN) and ex situ Auger electron spectroscopy (AES) measurements. The CV, EQCN and AES data show that the charge density, interfacial mass variation and intensity of the O-to-Pt AES signal ratio increase in a continuous, almost linear manner as the potential is raised from 0.85 to 1.40 V. In addition, the charge density, mass variation and O-to-Pt signal ratio profiles, follow each other, thus indicating that the surface oxidation proceeds by a progressive coordination of O-containing species to the Pt substrate. The coupled CV and EQCN measurements lead to in situ determination of the molecular weight of the interfacial species; these were identified as chemisorbed O (O-chem) at 0.85 less than or equal to E less than or equal to 1.10 V and as O2- in the form of a surface PtO at 1.20 less than or equal to E less than or equal to 1.40 V. The AES results reveal that the first half-monolayer of O-chem is formed through discharge of H2O molecules and such formed O-chem resides on the Pt surface. Subsequent discharge of H2O molecules leads to formation of the second half-monolayer of O,h,m that is accompanied by the interfacial place exchange Of O,h,,,, and surface Pt atoms; this process results in the development of a quasi-3D surface PtO lattice comprising Pt2+ and O2-. AES data demonstrate that the place-exchange process occurs in the 1.10 - 1.20 V potential range. The experimentally determined molecular weight of the species added to the surface is 15.8 g mol(-1), which points to O and to anhydrous PtO as the surface oxide formed. (C) 2003 Elsevier Ltd. All rights reserved.