The construction and sensing performance of solid-state potentiometric NOx sensors using a natrium superionic conductor (NASICON) as solid electrolyte are reported. Dense ceramic pellets with a pure NASICON crystal phase were produced by a solid-state reaction method using a modified procedure. The NASICON surface on the Pt sensing electrode side was modified with a NaNO3 layer as an auxiliary phase using a melting-quenching method. By the same method, the other surface of the NASICON was modified with an AgCl layer and was connected with an Ag wire as a reference electrode. An air/Pt quasi-reference electrode was fabricated by exposing the Pt black attached NASICON surface to the ambient. Both sensor devices using air/Pt and Ag/AgCl reference electrodes were investigated for NOx sensing. The two sensor designs exhibited Nernstian behavior, fast response, stability, and reproducibility. The Ag/AgCl reference electrode provides an all-solid-state design for the sensor device without the need for a reference gas with fixed composition or a hermetic seal separating the reference electrode from the sample gas. A NOx sensing mechanism is discussed. (C) 2004 The Electrochemical Society.