When the electrochemical cell arrangement of type III, O(2), CO(2), Au, Li(2)CO(3)\Li(+) conductor\O(2), reference (a(Li2O) = constant) is used to measure the CO(2) concentration in air, the emf should depend only on the logarithmic concentration of CO(2). In reality, however, the sensor response is also affected by the oxygen present in the environment. The oxygen dependence of the CO(2) sensing electrochemical cell originates from an influence of electronic conduction in the electrolyte. The theoretical change of emf with oxygen pressure, which is predicted from the modified emf-equation for the electrochemical cell with a mixed conducting electrolyte, agrees well with the measured emf change. (C) 2002 Elsevier Science B.V. All rights reserved.