Au‐interfacial oxide layer (GeO2, Sb2O3, Bi2O3, SnO2and native oxide mixture of AS2O3and Ga2O3)‐semiconductor (GaAs) structures were investigated by the Auger Electron Spectroscopy Method. The results of depth profiling with Ar+‐ion sputtering are presented for all metal‐insulator‐semiconductor (MIS) structures. ’’Metal’’ atoms from deposited interfacial oxide layers (Ge from Ge2O3, Sb from Sb2O3, Bi from Bi2O3, and Sn from SnO2) were observed on the surface. Only As atoms were observed for the native oxide mixture of As2O3and Ga2O3interfacial layer. These findings suggest that As2O3is the dominating oxide at the metal‐oxide interface for native oxide GaAs MIS solar cells. The interfacial reaction takes place between Au and the interfacial layer at room temperature. The ’’diffusion’’ of metal atoms from the interfacial layer towards the surface is suspected to play a role in degradation effect in GaAs MIS solar cells.