Several variables may account for the response of electrochemical skin surface PO2sensors to anesthetic gases: cathode material and size, pH of the electrolyte and membrane material. These variables cannot be chosen arbitrarily and their influence has been tested with two types of sensors. In one type (LSC), a large size cathode (mm range) and a membrane with low permeability for oxygen such as mono-axially oriented polyethylene is used. The other type (MC) contains one or more microcathodes (μ range) and a membrane which is highly permeable for oxygen, such as Teflon PTFE.With the LSC sensor, the N2O interference current is smaller than 5% of the air current when the sensor is polarized at –600 mV. The interference current with 2% halothane is smaller than 3% of the air current. With the MC sensor, the N2O interference may be up to 40% of the current in air when the sensor is polarized at –800 mV. The magnitude of this interference depends considerably on the silver deposition on the platinum cathode. At –600 mV the N2O interference is negligible. However, at this polarization voltage, the sensor is not operated within the limiting current plateau of oxygen. The interference current with 2% halothane may be up to 30% of the current in air. With both types of sensors there was no measurable interference by 2% enflurane. The authors conclude that to reduce the interference of anesthetic gases at skin surface sensors for oxygen to a reasonable level, it is necessary to use a membrane with low permeability for oxygen and a polarization voltage of approximately –600 mV. These two conditions can be fulfilled optimally only with a sensor design in which a large size cathode is used.At Stowe-Severinghaus type skin surface sensors for PCO2, there is no measurable interference by N2O, halothane or enflurane.