Solid-state electrochemical sensors using two different sensing electrode compositions, gold and strontium-doped lanthanum manganite (LSM), were evaluated for gas-phase sensing of NOx (NO and NO2) using an impedancemetric technique. An asymmetric cell design utilizing porous yttria-stabilized zirconia (YSZ) electrolyte exposed both electrodes to the test gas (i.e., no reference gas). Sensitivity to less than 5 ppm NO and response/recovery times (10-90%) less than 10 s were demonstrated. Using an LSM sensing electrode, a virtual identical sensitivity toward NO and NO2 was obtained, indicating that the equilibrium gas concentration was measured by the sensing electrode. In contrast, for cells employing a gold sensing electrode, the NOx sensitivity varied depending on the cell design: Increasing the amount of porous YSZ electrolyte on the sensor surface produced higher NO2 sensitivity compared to NO. To achieve comparable sensitivity for both NO and NO2, the cell with the LSM sensing electrode required operation at a lower temperature (575 degrees C) than the cell with the gold sensing electrode (650 degrees C). The role of surface reactions is proposed to explain the differences in NO and NO2 selectivity using the two different electrode materials.