The formation of highly oxidative and reductive states within and around WO3 and In2O3 during detection of NOx (NO and NO2) was investigated by attaching the metal oxide electrode to a proton-conducting Sn0.9In0.1P2O7 electrolyte, followed by electrode polarization at 200 degrees C. Proton insertion and release occurred at the WO3 electrode, while metal-ion redox reactions proceeded at the In2O3 electrode. Although the hydrogen tungsten bronze formed under cathodic polarization was insensitive to NO and NO2, the WO3 operating under anodic polarization possessed sensitivity toward NO and NO2. Addition of these gases increased the ohmic, charge-transfer, and gas-diffusion resistances in a similar manner. The In2O3 underwent significant changes in NOx sensing ability upon changes in polarity and magnitude of the applied voltage. In particular, under cathodic polarization, the sensor resistance decreased upon addition of NO and NO2 in the range of -0.5 to -1.5 V, but increased upon addition at -2.0 V, due to reduction of In3+ to lower valence states, which selectively absorbed NO2 beginning at -1.7 V. The WO3 and In2O3 sensing abilities were not significantly affected by interference from O-2, H-2, CO, or n-C4H10, and the sensors became more sensitive toward NO2 as the WO3 and In2O3 particle size decreased. (C) 2017 The Electrochemical Society. All rights reserved.