An instrument for measuring gas and flame temperatures, is described. From design considerations it is shown that the instrument can be used at higher temperatures than existing probe instruments and has a very rapid rate of response. The operation of the instrument is discussed and examples of its use are given. In the downtakes of open-hearth furnaces and in water-tube boilers the instrument shows advantages over the suction pyrometer in speed of response and freedom from blockage. Results have also been obtained at temperatures higher than those at which any other method of obtaining local temperatures has proved successful.