The chemical bonding state and surface morphology of sulfuric-vapor treated GaAs surface were investigated using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The sulfuric vapor was obtained through the heating of (NH4)(2)S-x solution at 60 degrees C. At the initial state of the treatment, the decrease of elemental As and the generation of As-S bonds were observed. Even after all of the elemental As converted into As-S bonds, peak area of the As-S bond increased until 5 min exposure. Unlike the liquid-phase treatment, the dissolution of the elemental As did not happen during the treatment and the formation of As-polysulfides was observed. Through the surface treatment of the GaAs using sulfuric vapor, small particles were formed and increased with treatment time. After 5 min exposure, they were linked to form a large agglomerate. This corresponds to a deposit of polysulfides containing hydrogen. To evaluate the effectiveness of the sulfuric vapor-treatment against oxidation in air, the treated GaAs was exposed to air and compared with the GaAs treated using a (NH,)IS, solution. The As-polysulfides and polysulfides containing hydrogen were revealed to be easily decomposed by exposure to air. The character of S bond with GaAs was revealed to be the most important to obtain a passivation effect.