Here, we show that residual contaminants in purified single-wall carbon nanotube bundles may be responsible for the reported sensitivity of the electronic and transport properties to oxygen. Removal of these contaminants makes the electronic spectra insensitive to O-2, CO, H2O, and N-2, while a strong sensitivity to NO2, SO2, and NH3 is observed, confirming the possible application of single-wall nanotubes as powerful sensors capable of measuring environmentally significant levels of toxic gases.