Surface atomic composition and chemical state have been investigated for Si (100) bombarded with 5 keV ions (B⁺, C⁺, N⁺, O⁺, F⁺, Ne⁺). The XPS observations reveal that the tendencies of surface chemical change by the irradiation are divided into three types. These are, 1) formation of monatomic layer composed of bombarding ions (B⁺, C⁺), 2) formation of new compounds between silicon and bombarding ions (N⁺, O⁺), and 3) no chemical change observed (F⁺, Ne⁺). The changes in surface atomic compositions are discussed in connection with irradiation-induced surface segre-gation of the implanted atoms and free energy (ΔG) or sublimation energy of the Si compounds.