Nanostructured thin films of Ba-doped indium oxide have been examined for monitoring ppm levels of NOx in ambient air, and their response patterns have been recorded at different operating conditions. These films are found to be capable of sensing down to 0.5 ppm NOx concentration in air. The optimum operating temperature for sensing is found to be 300°C. Response and retrace times are typically 60 s and 300 s respectively. They are highly selective to NOx and do not show significant response to hydrogen and petroleum gas up to 5000 ppm.