To estimate internal doses due to the inhalation of radionuclides produced by the nuclear spallation of the air nuclei in high-energy proton accelerator facilities, the physicochemical properties of radionuclides are very important. Thus, the ratio of aerosol and gases of Cl-38 and Cl-39 formed by irradiating argon gas-added air with a 48 MeV proton beam has been measured. Radionuclides of Cl-38 and Cl-39 exist as aerosol, acid gas and non-acid gas. The percentages of activity of Cl-38 and Cl-39 aerosols are about 80%. The number size distributions of non-radioactive aerosol were characterised by two peaks with diameters of 10-20 nm and larger than 20 nm. As a result predicted by a simple surface model, it was found that the activity size distribution of Cl-38 aerosols can be regarded as that having a single peak at 120 nm.