The harmful nature of nitrogen oxides (NOx) to environmental systems necessitates sensors to detect their presence. One effective means is the use of solid state sensors. These sensors are compact, rugged, and can be inexpensively fashioned, making them a viable option for measuring gas concentration. A non-Nernstian potentiometric sensor can be used to detect low concentrations of NOx gas in multiple regions of oxygen concentration. Understanding the mechanism for this behavior can assist in optimizing the sensor couple for a given application. Previous studies using Temperature Programmed Reaction and Desorption (TPR/TPD), along with IR and XPS analysis, have identified the formation of charge-building compounds that establish voltage on the sensing electrode when exposed to NOx gas. To further elucidate the mechanism, TPR and TPD were performed using isotopically labeled oxygen with simultaneous exposure to NOx. The material was examined under multiple gas conditions of NOx and O-16(2)/O-18(2) atmospheres, as well as varied levels of O-18 enrichment in the lattice itself. Through these studies, it was determined that the formation of charged surface complexes occurs solely through the use of lattice oxygen (vs. gas phase O-2) with adsorbed NOx. (C) 2008 Elsevier B.V. All rights reserved.