The physicochemical properties of radionuclides suspended in the air are important parameters in order to evaluate internal doses due to the inhalation of the airborne radionuclides and to develop the air-monitoring system in high-energy proton accelerator facilities. This study focuses on the property of radioactive airborne chlorine (Cl-38 and Cl-39) and sulphur (S-38) produced in Ar gas by irradiation with high-energy neutrons. As a result of the irradiation of a mixture of Ar gas and dry air, Cl-38 and Cl-39 existed as non-acidic gas and S-38 was present as acidic gas. Furthermore, it has been found that in the high-energy neutron irradiation of aerosol containing Ar gas, the higher the amount of radioactive aerosols, the lower will be the amount of radioactive acidic gas.