On the surfaces of crystalline semi-insulating GaAs (100), irradiated with noble gas Ar+ and Xe+ layers of native oxides are formed in the air under ordinary conditions. The surface atomic density of elements and chemical composition of those near the surface layers were studied by means of Rutherford backscattering spectroscopy, nuclear reaction and X-ray photoelectron spectroscopy. In these layers the dependences of the surface atomic density of oxygen and chemical compositions as a function of doses in the region from 1 x 10(13) to 1 x 10(16) ions/cm(2) were determined. It was found that the former value depends on the kind of implanted ions and the relationships of Ga2O3, As2O3 and As2O5 in the native oxide layers changed with the increase of the fluence. Moreover, the concentrations of Ga2O3 increased and those of As2O3 and As2O5 decreased with the increasing irradiation fluence. (C) 2016 Elsevier B.V. All rights reserved.