An amperometric solid-state sensor for the detection of nitrogen oxide in air has been constructed using a gold minigrid indicator electrode, Pt/air reference and platinum counter electrodes in contact with a solid polymer electrolyte containing 9.6% PVC, 87.0% 2-nitrophenyl octyl ether and 3.4% tetrabutylammonium hexafluorophosphate. The stationary electric current corresponding to the NO 2 reduction at the indicator electrode at -0.3 V versus Pt/air electrode exhibited a linear dependence on the NO 2 concentration in the range 0.2 to 5.0 ppm, with a sensitivity of 21.7 nA ppm - 1 (at 54% relative humidity), a standard deviation of 3.3 nA at 1.06 ppm NO 2 , and a detection limit of 75 ppb. The time constant or time required to attain 90% of the steady-state response is 3.3 s or 14 s, respectively. The response decreases linearly with increasing relative humidity (RH) with a slope of 0.14 nA ppm - 1 per 1% RH. The sensor shows an excellent stability over one month.