The chemical composition and the electronic properties of the n-GaP(100) surface treated with ammonium sulfide dissolved in water versus 2-propanol is investigated using synchrotron-radiation photoemission spectroscopy (SXPS). Chemical composition and electronic structure of the passivated surfaces vary in dependence of the solvent used. The valence band maximum and the core level binding energies of the sulfide layers measured in the most surface sensitive mode are found at 1.0 and 0.75–0.9 eV higher binding energies using 2-propanol versus aqueous solution. This flattening of bands actually is related to photovoltage induced by synchrotron radiation. The strong source induced photovoltage obtained after treatment with 2-propanol solution indicates a high quality surface as surface recombination is largely suppressed due to passivation of surface gap states. In addition the carbon contamination is reduced three times using alcoholic while it is increased by a factor of 1.4 with aqueous solution.