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The chemical surface state of polished GaAs (100) wafers after treatments in hydrofluoric acid and in water were investigated by means of x‐ray photoelectron spectroscopy (XPS) and high‐resolution electron energy‐loss spectroscopy (HREELS). After the HF treatment, the wafer surface is As rich and partially covered with fluorine. The fluorine is bonded to Ga atoms as can be deduced out of a chemically shifted component in the XPS spectra. We cannot detect hydride groups using vibrational spectroscopy (HREELS). Fluorine on top of the wafer surface can be removed by a short water rinse. Depending on the storage time of GaAs wafers in HF the As enrichment of the surface increases. We present a model for the surface state after the HF treatment with an As overlayer on top of bulk GaAs. Consistent with our angular dependent XPS measurements, we can give values for the thickness of the As overlayer. Conditions of illumination and concentration of HF influence the reaction rate of HF on the GaAs surface. The reaction with water depends strongly on the illumination and whether the treatment in water is performed in static or dynamic conditions. Rinsing samples in running de‐ionized water and dark storage in water results in an oxide removal. Light storage in water in static conditions yields a Ga‐rich oxide with the growth rate depending on the illumination. The oxide growth is faster on n‐type GaAs than on p‐type.