Using the galvanic cell: Pt\Au,CO2,O-2, \(aNa''') Na2CO3(Au) \(aNa'') NASICON \('aNa) FeO+NiO(borate glass)\FeNi(48)\Au\Pt the p-electronic conduction parameter a(circle plus) of NASICON was characterized as a function of the sodium activity a(Na)(") and the temperature. For N that purpose, the isothermal voltage response to successive variations of a(Na)(") by changing the composition of the CO2-O-2 gas mixture was N evaluated by a non-linear regression procedure. Within the temperature range under investigation (300-620degreesC) it holds that: log a(circle plus) = 11.076 +/- 0.1 - (18278.0 +/- 11000) K/T (T less than or equal to 500degreesC) log a(circle plus) = -3.519 +/- 1.7 - (7904 +/- 1400) K/T (500degreesC less than or equal to T < 620degreesC) Thus, in a CO, sensor with NASICON as solid electrolyte the sodium activity prevailing at the measuring electrode is outside the ionic domain of the electrolyte. This is in apparent contradiction to most of the literature reports on the behavior of such gas sensors, but confirms previous findings on the extent of the electronic conductivity in Na-beta-alumina under comparable conditions. (C) 2004 Elsevier Ltd. All rights reserved.