An amperometric NO 2 gas sensor based on Pt/Nafion ( R) electrode has been prepared using an impregnation-reduction chemical plating method and tested over the NO 2 concentration range from 0 to 485ppm. A Nafion ( R) membrane was used as a supporting material and Pt/Nafion ( R) electrode as an anode maintained at a fixed potential, diffusive regions (or mass transfer controlled regions) with respect to a reference electrode of preferably 0.95-1.10V. A 45s response time (t 9 5 ) was attained with a 0-485ppm step change in NO 2 concentration whereas the recovery time (t 9 5 ) was 54s when the NO 2 concentration was stepped from 485 to 0ppm. The sensitivity for the NO 2 (0-100ppm) gas sensor was 0.16μA/ppm (52nA/ppm/cm 2 ) at room temperature. The NO 2 sensor, based on a Pt/Nafion ( R) electrode, showed a long-term stability greater than 27 days, suggesting its possible applicability for environmental monitoring.