We employed density functional theory to characterize CO2 adsorption on BNNT and AlNNT surfaces. The effects of S functionalization on the adsorption of CO2 gas on BNNT and AlNNT surfaces were investigated. Results reveal that adsorptions of CO2 on studied nanotubes were exothermic and experimentally possible from the energetic viewpoint. Results show that, Ead values of CO2 on AlNNT surface were more negative than corresponding values of BNNT. Results reveal that, S functionalization of studied nanotubes causes an increase in the absolute values of Ead of CO2 on surface of studied nanotubes. These results show that, there are good linearity dependencies between Ead and orbital energy values of studied nanotubes. Therefore we can conclude the Ead and orbital energy values are highly sensitive to the adsorption process which these may be used for the selection the suitable nanotubes with enhanced CO2 adsorption potential. (C) 2016 Elsevier B.V. All rights reserved.