In situ core level photoelectron spectroscopy and mass spectrometry have been utilized to study the methanol oxidation on a model RuO2 catalyst at pressures ranging from 10(-6) to 10(-1) mbar. The experiments were carried out varying the O-2/CH3OH molecular mixing ratio from 0.25 to 3.3 and the reaction temperature from 350 to 720 K. The Ru 3d(5/2) and O is core level spectra were used to characterise the dynamic changes in the Ru oxidation state by exposing the oxide pre-catalyst to different reagents partial pressures and temperatures. Full oxidation to CO2 + H2O or partial oxidation to CO + H2O + H-2 have been observed in the whole pressure range for specific reaction conditions, which preserve the oxide catalyst state or reduce the oxide to metallic Ru. The selective oxidation to formaldehyde is observed only at pressures in the 10(-1) mbar range, catalyzed by a RuOx surface oxide formed by partial reduction of the oxide pre-catalyst. (c) 2007 Elsevier B.V. All rights reserved.