This work studied the adsorption of argon and krypton onto Single Walled Carbon Nanotubes (SWCNTs) at liquid nitrogen temperature (77 K). The volumetric method was used to characterize our sample after purification by nitrogen adsorption. This method was also used to examine the effect of the size of molecules on the adsorption properties (energy and adsorption capacity) of SWCNTs and determined the adsorption sites. The results obtained show that the specific surface of the sample almost doubled after purification. This research showed that the energy and the adsorption capacity decrease when the molecule size increases; small molecules have access to all the adsorption sites of SWCNT and those of larger size do not adsorb in the lower interstitial channels. Finally, the studies have shown that the krypton does not adsorb in the micropores.