The open-circuit voltage (e.m.f.) of the solid-state potentiometric sensor for CO2, Pt, P-CO2(I)/Na2CO3//Nasicon(Na+)//P-CO2(I)/Au-Pd, Pt has been measured at 650, 690 and 720 K as a function of the partial pressure of CO2 between 150 and 10 000 ppm. Nasicon is used as a solid sodium ion conductor and pure Na2CO3 is employed as an auxiliary electrode. The e.m.f. of the sensor is measured in dry and moist gas mixture containing water vapour at a partial pressure of 2.4 kPa. The measured e.m.f. of the sensor was found to be unaffected by the presence of water vapour in the gas mixture. The e.m.f. of the sensor was found to be Nemstian and was reproducible within +/-5 mV on cycling through the range of partial pressure of CO2 employed in this study.