AES and EELS are used to study the effect of oxygen on the FeS2 surface. The main Auger Fe M2.3VV attenuates with increased oxygen exposures and shifts to lower kinetic energy while the high kinetic energy feature due to an 'autoionization process' remains unaltered in energy. The EELS spectra measured as a function of oxygen exposure clearly indicate the formulation of iron oxide similar to that of alpha -Fe2O3. For the FeS2+O2 system, the authors show that U eff is negligibly small and is in good agreement with the estimated Ueff for Fe2O3. A slight change in the S L 2.3VV Auger peak is observed on adsorption of oxygen and the formation of sulphur oxides is ruled out.