A study on the interface properties of Schottky contacts on (NH4)2Sx-treated n- and p-type In0.5Ga0.5P is carried out. The effects of sulfur (S) treatment on Schottky barrier height are investigated by employing capacitance-voltage and current-voltage (I-V) measurements. It is also demonstrated that the passivation effects of S treatments on the interface traps can be monitored by deep level transient spectroscopy (DLTS) measurements. It is observed that the S treatment increases the dependence of Schottky barrier height on the metal work function. The interface traps in the Schottky contact formed by the heat treatment are found to give their energy state above midgap. It is found that the S treatment can passivate these interface traps as well as suppress their generation under the heat treatment. For both n- and p-In0.5Ga0.5P, contact-related majority carrier traps, which are different from the thermally generated interface traps, are observed at the Al-In0.5Ga0.5P interface and they can be annealed out by a heat treatment at 350 oC. It is also found that the I-V characteristics of Au/InGaP diodes formed on the S-treated surface degrade more rapidly than those formed on the untreated surface. Through cross-sectional transmission electron microscope observation, this poorer electrical reliability of Au contact on S-treated surfaces is attributed to the enhanced intermixing of group III elements with Au.