The adsorption of small molecules (N2, O2, CO, CO2, H2O, and HF) in model ropes of carbon nanotubes has been studied to determine the main parameters (stable adsorption sites, potential barriers, …) which define the ability of carbon nanotubes to select small molecules through their different behavior in the diffusion mechanism. When the polarization of the nanotubes is taken into account in the semiempirical potentials, it has a significant influence on the adsorption of polar species. Examination of the potential maps along the ropes shows that the nature and the stability of the adsorption sites are strongly dependent on the molecular species and on the diameter of the tubes. For a small rope formed with (10,10) single-wall nanotubes, different trapping sites are favored by the molecules considered. Furthermore the corresponding trapping well depths are sufficiently selective to discriminate the species. Improving the size homogeneity of the ropes and judiciously calibrating their diameter would provide an efficient mean of selecting molecular species.