We investigated the molecular adsorption and desorption on single-walled carbon nanotubes (SWCNTs) by thermal-desorption spectroscopy (TDS) and Fourier transform infrared spectroscopy (FTIR). By examining the as-purified and air-oxidized SWCNTs, the adsorption of H₂, N₂ and CO on the groove, inside and interstitial channel were clearly identified. In the case of N₂ adsorption at low temperature, occupation of the inside site was found to be diffusion-limited. Co-adsorption of H₂ and N₂ at different temperatures revealed that the groove and inside sites are selectively occupied by either of the two molecules. FTIR spectra of CO on SWCNTs exhibited two distinct features for the CO stretching mode depending on the adsorption site.