Ballistic Electron Emission Microscopy (BEEM) was used to investigate the effects of a sulfide interfacial layer on the electronic properties of the Au/n-GaAs interface. Two diodes were simultaneously prepared in UHV by evaporating Au onto (100)GaAs substrates which were either treated with (NH4)2S or NH4OH etching solutions prior to Au deposition. Auger electron spectroscopy was used in situ to monitor the surface chemical composition. The effective Schottky barrier (SB) height was measured with BEEM at 10 different locations on the surface of each diode. The (NH4)2S treatment increased the average barrier height and reduced the spread in the values, as compared to the contact formed on the NH4OH treated GaAs.