In this article, we have reported the dependence of electrical conductance on the presence of structural defects and of molecular-oxygen adsorbates in carbon nanotube thin films for NO2 detection. Our results show that an oxygen contaminant may be responsible for the reported sensitivity to NO2. Experimental results indicate that the adsorption and the uptake of the oxidizing gas are more likely to occur as a result of the interaction between the O-2 molecule and defective sites in the hexagonal network of the nanotubes. Experimental findings demonstrate that nanotubes could be used as sensitive chemical gas sensors and likewise indicate that the intrinsic properties measured on as grown nanotubes may be changed by extrinsic oxidative effects. (C) 2004 American Vacuum Society.