A portable detector for mixtures of nitric oxide and nitrogen dioxide was developed using a porous silver cathode in sulfuric acid supporting electrolyte. Nitrogen dioxide was determined in the gas stream by electro-reduction. The nitric oxide in the gas stream was oxidized to nitrogen dioxide by passing it over cr (VI) on firebrick. Electroreduction gave the sum of no + NO2. Analog circuitry was used to display the amounts of no and NO2 separately. Two ranges available for each gas were, in parts per million, 0 to 10 and 0 to 50 for no, and 0 to 2 and 0 to 10 for NO2.