In this paper, by the use of X-ray photoelectron spectroscopy, we unequivocally identify the oxides present on GaAs surfaces and accurately measure the binding energies associated with the 2p(3/2), 3d, and Auger lines in the X-ray photoemission spectra. These measurements intended to provide reliable reference data for further work. We conducted an extensive analysis of the oxidation states of Ga metal and oxide powder reference samples. air exposed GaAs wafers, and wafers subjected to various surface treatments (argon plasma treatments and boiling). Based on this experimental evidence, an assignment of the photoelectron peaks to various chemical states is proposed.