An impedancemetric method for NOx sensing using an yttria-stabilized zirconia (YSZ)-based electrochemical cell is described. The sensor cell consists of a planar YSZ electrolyte and two identical YSZ/Cr2O3 composite electrodes exposed to the test gas. The sensor response to a sinusoidal ac signal applied between the two electrodes is measured via two parameters calculated from the complex impedance, the modulus vertical bar Z vertical bar and phase angle Theta. While either of these parameters can be correlated to the NOx concentration in the test gas, Theta was found to provide a more robust metric than vertical bar Z vertical bar. At frequencies below approximately 100 Hz, Theta is sensitive to both the NOx and O-2 concentrations. At higher frequencies, Theta is predominantly affected by the O-2 concentration. A dual frequency measurement is demonstrated to compensate for changes in the O-2 background between 2 and 18.9%. Excellent sensor performance is obtained for NOx concentrations in the range of 8-50 ppm in background. An equivalent- circuit model was used to extract fitting parameters from the impedance spectra for a preliminary analysis of NOx-sensing mechanisms. (c) 2007 The Electrochemical Society. All rights reserved.