Grand canonical Monte Carlo (GCMC) simulation is used to model adsorption of CO ( 2 ) and SO ( 2 ) molecules by the pure and functionalized single walled carbon nanotubes (SWCNT and FSWCNT). Simulations are conducted for two cases: i) without and ii) with a 5.75 angstrom clearance between the CNT wall and the walls of the simulation box. Results show that the adsorption capacity for the SO ( 2 ) molecules is generally higher than that of the CO ( 2 ) molecules, especially at low pressures. Although for both molecules, the inside adsorption is higher at low pressures, the outside adsorption becomes larger above 10 and 2bar for CO ( 2 ) and SO ( 2 ), respectively. At 5bar, the total amounts of adsorption for case (ii) is higher by similar to 42% and similar to 271% for CO ( 2 ) and SO ( 2 ), respectively, compared with those for case (i) . Functionalization does not show significant effect on the adsorption of the CO ( 2 ) and SO ( 2 ) gases. Adsorption of CO ( 2 ) and SO ( 2 ) gases in the FSWCNT are rather more sensitive to the parameters of the force field, especially those used for the functionalized carbon atoms, than the number of carbon atoms affected by functional groups. The effects of non-spherical interactions on the adsorption isotherms have also been examined.