Three well-defined adsorption sites have been found on opened single-wall carbon nanotubes by temperatureprogrammed desorption measurements for several alkanes. A series of linear chain alkanes from pentane to nonane, as well as a branched alkane molecule, 2,2,4-trimethylpentane, were used to elucidate the effect of molecular length on the capacity of the adsorption sites. The two highest-energy adsorption sites were assigned as the nanotube interior sites and groove sites on the outside of the nanotube bundles. Hybrid Monte Carlo simulations were performed to probe the molecular-level details of adsorption. Both in experiments and in the simulation, the groove sites were seen to behave as one-dimensional adsorption space, demonstrating an inverse dependence of capacity on the length of the adsorbed molecule. In contrast, the capacity of the internal sites was found to depend inversely on the volume occupied by the molecule.