A fast P02 tracing with 95% delay times of 15–20 msec in routine is necessary for measurement of the PO, jumps in the expired air, particularly for the precise measurement of the so-called dead-air plateau. A Clark type PO, electrode has been developed which has the demanded property. This electrode has four characteristic features: (1) a glass-coated Pt cathode with a tip diameter of 0.3 mm (diameter of the Pt wire = 0.1 mm); (2) a differential screwdrive for feeding the cathode 50 μm per turn; (3) a 0.2- to 0.3-μm-thin polytetrafluoroethylene membrane, and (4) a special polishment of the surface of the cathode. The electrode has a 95% response time of 15–20 msec in routine when tracing PO2 changes from 150 to 750 mm Hg. About 30% of the prepared electrodes have response times of 7–10 msec. The output of the electrode in air is stable within the first 3 h (deviation smaller than 0.2%) and then decreases within 10 h approximately 1% per hour. The response times are constant for more than 48 h. The output of the electrode is linear from 0 to 760 mm Hg PO2. Within the accuracy of measurement the output is independent of PCO