A potentiometric NOx sensor using a proton-conducting Sn0.9In0.1P2O7 electrolyte with a PtRh/C working electrode was fabricated to study sensing properties for NO and NO2 at intermediate temperatures. The sensor showed electromotive force (emf) responses to changes in NO and NO2 concentrations. Interestingly, the emf values for both NO and NO2 increased in the positive direction with increasing gas concentration. The sensing mechanism was shown to be based on a mixed potential at the working electrode through measurements of the polarization curves of NO or NO2 and water vapor.