Interface chemistry of H2O on GaAs nanowires is investigated by in situ X-ray photoelectron spectroscopy (XPS) at elevated water vapor pressures (from UHV to 5 mbar) and temperatures (from room temperature to 400 degrees C). Surface-assisted water dissociation leads to oxidation and hydroxylation of surface Ga atoms. In comparison with the simple planar GaAs(1 0 0) crystal, the H2O dissociation process on GaAs nanowires is greatly enhanced at elevated pressures and temperatures. (C) 2014 Elsevier B.V. All rights reserved.