The stability of the SrZr0.9Yb0.1O3-alpha protonic conductor in an atmosphere containing nitrogen oxides (NOx) was investigated. When a fine powder of SrZr0.9Yb0.1O3-alpha with a specific surface area of about 50 m(2)/g was annealed at 440 degrees C in He gas containing 8% O-2 and 0.1% NO, the formation of Sr(NO3)(2) was observed by IR measurement, ion-chromatography analysis and ICP analysis. The formation mechanism of Sr(NO3)(2) was examined by considering the thermodynamic equilibrium. Based on the results of the thermodynamic calculation, H2O dissolved into SrZr0.9Yb0.1O3-alpha was estimated to play an important role in the reaction for the formation of Sr(NO3)(2) between SrZr0.9Yb0.1O3-alpha and NOx.