prepared. The ionic conductors used were yttria-stabilized zirconia that is an oxygen ion conductor and sodium supersonic conductor whose charge carriers are sodium ions. Thick films of LaFeO3 powders were deposited using a screen printing oil on one side of the solid electrolyte pellets, using powders having different grain sizes. The obtained sensors were wholly exposed to the same atmosphere. Electromotive force (emf), polarization curves, and amperometric measurements were performed in air and at different concentrations. of NO2, in air, at selected temperatures. Quite stable emf values and fast response times were obtained. The NO2 response was much larger when powders with nanosized grains were used for the preparation of the electrodes. From polarization curves, all sensors showed a nonlinear behavior. The role played by the electrolyte/electrode interface was determined by electrochemical impedance spectroscopy measurements; exposure to NO2 did not affect the bulk but only the electrolyte/electrode interface.