GaAs surface chemical passivation by wet chemical treatments in hydrazine-sulfide solutions with different pH values has been studied by X-ray photoelectron spectroscopy and spectroscopic ellipsometry. It is found that highly alkaline (pH 12) solutions result in a coherent monomolecular film of gallium nitride, while treatment in buffered solutions (pH < 8) yields a surface layer of gallium sulfide. Both nitride and sulfide surface films are found to be stable against oxidation in air ambient over months, attesting to the validity of the wet chemical treatment in hydrazine sulfide solutions for chemical passivation of the GaAs(100) surface. The chemical mechanism for formation of the surface nitride and sulfide films is presented and discussed. (c) 2005 The Electrochemical Society.