We report on Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS) investigations on ErSi2 : the silicide behaviour under Ar+ bombardment (1-5 KeV) and oxygen exposure at room temperature was studied. The ion beam processed surface shows a Si enrichment, resulting in an ErSi-like stoichiometry. This result suggests a predominant role of the mass difference between Er and Si in the sputtering mechanism of ErSi2 At exposure up to 103 langmuirs, both components of the silicide react with oxygen. Firstly, Er oxide is formed, in an Er2O3-like state. After the consumption of the available Er atoms in the surface layer, SiO2 starts to grow. These results are interpreted in terms of a greater heat of formation for Er oxide than for Si oxide.