NO x sensors composed of partially stabilized zirconia (PSZ), fully stabilized zirconia (FSZ), and PSZ-FSZ composite electrolytes were investigated using impedance spectroscopy under dry and humidified gas conditions. The impedance data were used to interpret the electrochemical behavior of the various sensors as the water concentration in the gas stream varied. The sensors were operated in the presence of 0-100 ppm NO with 1-18% O 2 and 3-10% H 2 O with N 2 as the balance gas. The operating temperature of the sensors ranged from 600 to 700 C. The impedance response for sensors containing ≥ 50 vol% PSZ slightly decreased under humidified gas conditions, in comparison to dry gas conditions; whereas, a significant increase in impedance occurred for sensor largely containing FSZ. This indicated water cross-sensitivity was substantial at FSZbased sensors. The microstructural properties, NO x sensitivity, oxygen partial pressure and temperature dependence, as well as the response time of the sensors composed of the various electrolytes were characterized in order to interpret the electrochemical response with respect to water cross-sensitivity. Analysis of the data indicated that sensors composed of a PSZ-FSZ composite electrolyte with 50 vol% PSZ were more suitable for detecting NO x while limiting water cross-sensitivity.