The effects of (NH4)2Sx solution treatment on the interface properties of metal-In0.5Ga0.5P Schottky contacts have been investigated by capacitance-voltage measurements and deep-level transient spectroscopy measurements. The (NH4)2Sx-treated samples show Schottky barrier heights that are more sensitive to the metal work functions. It is also found that (NH4)2Sx treatment of In0.5Ga0.5P can passivate the phosphorus-vacancy-related interface deep traps of Schottky contacts as well as suppress the generation of interface deep traps due to heat treatment.