Adsorption isotherms of methane and ethane in single-walled carbon nanotubes (SWNTs) were measured by 1H nuclear magnetic resonance (NMR) at room temperature. It is shown that the interior of SWNTs becomes available for methane and ethane adsorption after cutting of SWNTs. Such endohedral adsorption dominates methane and ethane adsorption in SWNTs, at least below 1 MPa. The average exchange time between molecules adsorbed inside SWNTs and free gas molecules outside was estimated to be on the order of 80 ms. It is shown that exposure to oxygen has no effect on methane and ethane endohedral adsorption in SWNTs, suggesting that the adsorption energy of oxygen molecules inside SWNTs is small compared to that of methane. 13C NMR indicates that under atmospheric pressure and room temperature helium atoms could access the interstitial sites of SWNT bundles whereas H2, CO2, and N2 molecules could not
