Perovskite-type oxide, La0.5Sr0.5CoO3-delta, has large oxygen nonstoichiometry and high electrical conductivity with a large carrier concentration. The oxygen nonstoichiometry, delta, of dense La0.5Sr0.4CoO3-delta was measured in the stream of NOx-O-2-N-2 system at 873 to 1073 K by means of a thermogravimetric method. Small amounts of NO2 caused large changes in oxygen nonstoichiometry. It seems probable that excessive amounts of oxygen can be incorporated into the bulk due to catalytic decomposition of NO2, and as a result, oxygen potential on the surface of the sample seems to be much higher than that of gaseous phase. EMF measurements for dense La0.6Sr0.4CoO3-delta electrode also revealed that the oxygen potential is affected by NO2. Thus, the gas sensing mechanism of this oxide was found to relate to the change of the bulk property, especially oxygen nonstoichiometry. (c) 2005 Elsevier B.V. All rights reserved.