Surface properties of GaAs passivated with (NH4)2Sx solution have been compared with HCl-treated GaAs using X-ray photoelectron spectroscopy, Sulfur treatment on GaAs surface results in the formation of S-Ga and S-As bonds, which remain after successive rinsing for I minute in DI water. The evolution of Ga 2p3 and As 3d peaks in the sulfidation treated GaAs was monitored with the exposing time to air. After 10 days exposure to air, the Ga-O and As-O bonds slightly increased, but maintained almost constant for further exposure. The increase of Ga-O and As-O bonds induces the partial decomposition of sulfur bonds. Decomposition and evaporation behaviors of sulfur and oxygen were observed through the heat treatment of sulfidation treated GaAs under ultra high vacuum (less than l × 10-9 torr). After anneal at 350 – 450 °C, slight decrease of sulfur and oxygen due to the decomposition of As-O bond were observed. No more sulfur was found after anneal at 550 – 650 °C, where the decomposition of Ga-O bond was completed.