The adsorption of Ar on open-ended single-walled carbon nanotube bundles was studied. The amount of argon, which corresponds to the first coverage adsorbed on open-ended carbon nanotube bundles, was about 3 mmol/g. The isosteric heat of adsorption was obtained from the isothermal adsorption measurement in the low coverage region performed at temperatures ranging from 120 to 135 K. The estimated heat of adsorption of argon in the low-coverage region was 182 meV and was comparable to that of nitrogen.