The cu-GaAS(ll0) interface formation nas been Studied with soft x-ray Photoemssion spectroscopy (SXPS) for Cu overlayers deposited at room temperature. Tne evolution of the tia 3a and As 3a spectra snow that strong interactions occur between Cu and the substrate during the formation of the Cu-GaAs interface. A cnemically smifted Ga 3d peak at 0.8 eV lower binging energy and strong moaification of the As 3d lineshape has been found. Detailed analysis nas shown that the dissociated As is preterentially segregated on the metal layer, but Ga remains mainly in the interfacial reion for trick Cu coverages (30 - 60 Å). Using a deconvolution tecnniqie we nave found that the final stabilized position of the interface Fermi level lies at about 0.9 eV below the conduction band minimum.