A CO.sub.2 sensor includes a body of solid electrolyte ceramic with alkali ion conductivity, and two electrodes of conductor material being inert with respect to the solid electrolyte ceramic. At least one of the electrodes is at least partially permeable, and the solid electrolyte ceramic is galvanically prepolarized. A measurement circuit configuration includes the CO.sub.2 sensor and a voltage measuring device coupled to the electrodes of the solid electrolyte ceramic body.