An electrochemical device combining an MgO-stabilized zirconia (MSZ) tube with an auxiliary phase of Ba(NO3)(2) was found to exhibit fairly good Nernst’s response to dilute NO2 in air at elevated temperature (450 degrees C). The response to NO in air could be improved to be roughly comparable to that to NO2 when Ba(NO3)(2) was added with 10 mol% CaCO3. The EMF response of the tubular device to a fixed concentration of NO2 or NO was strongly dependent on O-2 concentration. The effect of O-2 was much mitigated with a probe-type planar device.