The electrochemical oxygen pump has been studied using Ceo(0.8)Sm(0.2)O(1.9) as a solid electrolyte and gold metal as an electrode. The O-2 concentration in the flow of a mixture of 1000 ppm NO, 1-10% O-2, 5% H2O and 5% CO2 at a flow-rate of 20 ml min(-1) was electrochemically controlled to a constant value of 5% by applying anodic or cathodic currents to the cell at 900 degrees C. During oxygen pumping, NO was not electrolyzed at all and the pumping amount of oxygen obeyed Faraday’s law. A potentiometic- or amperometric-type sensor for NOx, when introduced into the resulting gas stream, exhibited an output signal without influence of the O-2 concentration.