In this Letter, variations in the transport characteristics of metallic single wall carbon nanotubes (SWCNTs) due to the absorption of NO2 and NH3 molecules on the surface are investigated using the Landauer formalism combined with the non-equilibrium Green's function techniques and ab initio electronic structure obtained using density functional theory (DFT). The electronic structure, charge distribution, transmission spectrum and I-V characteristics of the SWCNT are significantly changed on adsorption which is reflected in the conductance and transport characteristics of the SWCNT. Thus, we conclude that these systems can be used in CNT based NO2 and NH3 gas sensors. (C) 2014 Elsevier B.V. All rights reserved.