In this paper the experimental adsorption isotherm and density functional theory (DFT) are used to characterize a sample of single-walled carbon nanotube (SWNT) arrays. Because the adsorption in SWNT arrays is rather complicated, and will occur both inside and outside isolated nanotubes as well as in adsorption sites due to aggregations of nanotubes, it is necessary to introduce adsorption models with different mechanisms. Four adsorption models have been introduced and compared in this work. The pore size distributions, which correspond to the models adopted, are determined by minimizing the deviations between the experimental isotherm and the DFT calculation. Our results indicate that by introducing enough flexibility in model IV proposed, in which adsorption takes place in both the outer surface and inside of the tubes separately or simultaneously, can give a good fit to the experimental isotherm.