Poor dielectric properties of GaAs oxides are the drawback of the GaAs-based electronics preventing using these oxides as dielectric layers. The elemental and chemical compositions of the GaAs native oxide layer slightly irradiated by Ar+ ions with the fluence Q ~1 ∗ 1014 ions/cm2 have been studied by the synchrotron-based photoelectron spectroscopy. The effect of selective and total decay of arsenic oxides followed by diffusive escape of arsenic atoms from the oxide layer has been revealed. The effect results in three-fold Ga enrichment of the upper layer of the native oxide and in strong domination (~90 at%) of the Ga2O3 phase which is known to be a quite good dielectric with the bandgap width as wide as 4.8 eV. A band diagram was obtained for the native oxide nanolayer on the n-GaAs wafer. It has been shown that this natural nanostructure has a character of a p-n heterojunction. Display omitted •A selective ion-induced transformation of As2O3 into elemental As in GaAs oxide.•Threefold ion-induced enrichment in Ga in the GaAs oxide layer ~1 nm thick.•Ion-induced transformation of GaAs oxide into Ga2O3 dielectric.•A band diagram of the native oxide nanolayer on the n-GaAs wafer.