Abstract: The interface study on Au/GaSb and Au/GaAs at monolayer coverage in the UHV system by the Kelvin CPD method is presented. There are big jumps on well cleaved GaSb and GaAs surfaces after initial sub-monolayer gold deposition. It is consistent with the fact that the Fermi level is pinning at the surface states at the very beginning of Au deposition of about 0.2ML, according to the photoemission measurements. The experiments on Au/n-GaAs interface show that there are significant photo-effects on the CPD value, particularly in the case of less than 1 ML Au being deposited.