Abstract: An oxygen probe for measuring the carbon potential of endothermic gas in a heat treating furnace. The probe uses a readily available, automative-type oxygen sensor having a solid electrolyte element. The sensor is mounted on a heat conducting base plate. The sensor element is surrounded by a gas flow tube extending from the base plate. Exit ports around the base of the sensor element extend through the base plate and into the area defined by the gas flow tube causing an axial, radially symmetric flow of gas around the sensor element. The tip of the element is heated above the temperature of the gas while the base of the element is conductively cooled by the base plate. A temperature gradient is thus caused along the length of the sensor element, increasing its sensitivity.