A procedure conforming to its genre for regulating the temperature of a measurement sensor for determining an oxygen concentration in gas mixtures, in particular exhaust gases of combustion engines, is known from DE 198 38 456 A1, at which a detection voltage that corresponds with the oxygen concentration and that is supplied by the Nernst measuring cell is evaluated, whereby the measurement sensor is regulated to an operating temperature by a heating device and the actual operating temperature is determined from a measurement of an alternating current internal resistance of the Nernst cell. In order to eliminate manufacturing related variations of the resistance value this procedure determines an alternating current internal resistance of a feed line of electrodes of the Nernst cell when starting and/or re-starting the measurement sensor and by considering the determined actual alternating current internal resistance when determining the operating temperature. The determination of the operating temperature of the measurement sensor due to the internal resistance of the Nernst cell is only possible in a limited range, because the temperature characteristic line of the resistance of the electrolyte that creates the measurement sensor allows only a precise measurement in a limited temperature range due to its course. Furthermore the characteristic line provides a varying offset due to the feed line resistance. The characteristic line is also having errors by typographic inaccuracies.