A new sulfur passivation technique for GaAs surface by using S2Cl2 instead of (NH4)2Sx has been developed. Auger electron spectroscopy verifies the passivated surface is almost completely free of oxygen. The results of X-ray photoelectron spectroscopy indicate a relatively strong bonding between S and substrate atoms. A clean surface can be achieved by annealing in vacuum at 600°C. ZnSe thin films grown by hot wall epitaxy on S2Cl2 passivated GaAs show improved crystalline quality compared with those treated by conventional method, as indicated by Raman scattering measurements.