The chemisorption and the reaction of chlorine and hydrogen chloride on the GaAs(100)-4 x 6 surface at 110 K are investigated using soft X-ray photoelectron spectroscopy which employs synchrotron radiation. At low exposures, Cl-2 and HCl dissociate and preferentially adsorb on the As atom, which causes As-Ga bond breakage to initiate chlorination of the Ga atom. AsxCl is proposed to form at initial chlorination, in which the Cl atom is bonded to a high coordination site. Subsequently, various AsClx and GaClx (x = 1, 2, and 3) species are formed on the GaAs surface, and their corresponding chemical shifts are assigned. At high exposures, chlorination of the GaAs surface is saturated, and the surface is mainly covered with physisorbed Cl-2 and HCl molecules. Synchrotron radiation on the chlorinated GaAs surface stimulates both the photodesorption of Ga and As chlorides and the photodissociation of physisorbed Cl-2 molecules. A potential route for anisotropic cryogenic etching of the GaAs surface by the chlorine-containing compound under photon irradiation is discussed.