The role of porosity on impedancemetric NOx sensing was investigated for sensors composed of a porous Y-stabilized ZrO2 (YSZ) electrolyte and Au wire electrodes. NOx sensors were fabricated at firing temperatures of 950 degrees C, 1000 degrees C, and 1050 degrees C to establish different porous microstructures. Porosity calculations were determined from scanning electron microscopy images of the porous electrolytes using a three-dimensional statistical method. The mean porosity of sensors fired at 950 degrees C was 50.2%, and the mean porosity value decreased to 46.7% for sensors fired at 1050 degrees C. Impedance spectroscopy was used to measure the electrical response of the sensors while operating at 650 degrees C, in gas atmospheres of 0-100 ppm NOx and O-2 concentrations ranging from 1-18% in a balance of N-2. The impedance of the sensors demonstrated a strong dependence on porosity as a decrease in porosity of about 4% resulted in nearly a 50% decrease in the impedance. Analysis of the impedance data indicted NOx sensitivity increased as the YSZ electrolyte porosity decreased. The response to NOx concentrations <= 10 ppm were distinguishable at operating frequencies as high as 40 Hz enabling rapid sensing. (C) 2013 The Electrochemical Society. All rights reserved.