X-ray and UV photoelectron spectroscopy (XPS and UPS were used for studying the oxidation of Er films deposited on Ta substrate under ultra-high vacuum. Oxidation has been carried out at room temperatue with an oxygen pressure of 2x10(-5) mbar.Erbium exposition to oxygen produces; Er2O3 and a hydoxide. Subjecting the sample to a post annealing treatment at 750 degrees C dissociates the hydroxide and produces additional Er2O3. UPS and XPS valence band gives evidence that the formed oxide is Er2O3.