An apparatus and method for polarographically determining the amount of oxygen in a gaseous mixture containing interfering reducible gases such as nitrous oxide is disclosed. The invention has particular application to the determination of oxygen in nitrous oxide containing anesthetic gas mixtures. The oxygen content is determined by operating a special polarographic cell at about -0.6 volts, which is below the voltage at which nitrous oxide would be sensed polarographically. A special cathode is utilized in which the exposed cathode face is disposed near the periphery of a cathode support mounted centrally under the gas permeable membrane so that ion mobility is increased to decrease the internal resistance of the electrochemical cell. It is also found that at the low operating voltage the cathode tends to become passivated or inactive after use. In accordance with the invention the cathode is activated at turn-on by reversing polarity and applying a positive voltage pulse to the cell.