A stainless steel, self heated, flat surface, skin surface probe, covered with a Mylar membrane, admits gases diffusing through the skin surface into a mass spectrometer at rates sufficiently low to provide reasonable measurement of blood PO2 and PCO2. A second thermistor against the Mylar and insulated from the probe monitors skin temperature as an index of blood flow. Advantages of the mass spectrometer method compared to electrodes are that a single sensor detects all gases, including anaesthetics, the sensor is simple in design, the same mass spectrometer can monitor several patients simultaneously, anaesthetics do not interfere (as with halothane and the O2 electrode), comparative checks with blood gas may be done using the same mass spectrometer, and perhaps, by summing all gases it may be possible to determine how fully arterialized the skin is.