Abstract: Apparatus for analyzing the concentration of nitrogen oxides in a gas draws the gas through three separate housings in succession. Moisture is removed from the gas in the first housing and an oxidizing agent in the second chamber converts the nitrogen oxides in the dried gas into nitrogen dioxide. The third housing contains an electrolytic cell producing an electric signal proportional to the concentration of nitrogen dioxide in the converted gas. The electrolytic cell has an anode of activated carbon in a perforated jacket located in a neutral electrolyte and a cathode contacted by the converted gas. The cathode is composed of platinum tape wound onto a support. A sheath of glass fibres contacts both the cathode and the electrolyte and lifts the electrolyte by capillary action to wet the cathode.