When the electrochemical cell arrangement of type III, O 2 , CO 2 , Au, Li 2 CO 3 |Li + conductor|O 2 , reference (a L i 2 O =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.