This paper reports a study by X-ray photoelectron spectroscopy (XPS) and reflection electron energy loss spectroscopy (REELS) of very thin films of antimony oxide deposited on sputter-cleaned Al 2O 3 and SnO 2 substrates. The work aims the determination of the stability of the different oxidation states of antimony and the characterisation of the electronic interactions generated at the interface between substrate and deposited phase. Oxidation of the deposited moieties of the antimony oxide with a plasma of oxygen leads to the formation of a mixed valence compound consisting of a mixture of Sb 3+ and Sb 5+ oxidation states. A quantitative evaluation of this partially oxidised phase by XPS was not possible because the highest oxidised state of antimony was unstable in vacuum under X-ray radiation. Despite this effect, it was possible to show that the remaining Sb 5+ species depicted characteristic features in XPS and REELS spectra that can be clearly differentiated from Sb 3+ related contributions. Stoichiometric Sb 2O 3 deposits were obtained by direct evaporation of this oxide. The characterization of the Sb 2O 3/MO y systems revealed that the electronic parameters determined through photoemission (i.e., binding energy (BE) of the photoemission peaks and Auger parameter ( α ′)) are dependent on the amount of deposited oxide and on the type of substrate on which it is deposited. The changes, much larger on Al 2O 3 (ΔBE=0.7 eV, Δ α ′=−1.3 eV) than on SnO 2 (ΔBE=0.2 eV, Δ α ′=−0.2 eV), have been discussed in the frame of the Wagner plots and the chemical state vector concept. It is also deduced that the electronic and bonding interactions developed at the interface between substrate and deposited phase determine the observed variations of these parameters as a function of surface coverage. The results with the Sb 2O 3 model system show that the BE or α ′ values cannot be taken as unequivocal parameter to assign oxidation states of cations in deposited oxides.