Abstract The development of materials for high-temperature-steam piping is of considerable importance in view of the upward trend in power-plant operating temperatures. Chemical composition, steel-melting practice, and heat-treating procedure, and their effects upon physical properties and microstructure are important factors in appraising the probable behavior of a material in service. The metallurgist and power-plant engineer are particularly interested in the influence of these factors upon notch toughness of the material. Starting with the premise that there may be some correlation between notch toughness and performance of high-temperature pipe and that significance should be attached to the interpretation of notch toughness with respect to the behavior of carbon-molybdenum pipe material in power-plant service, the authors give details of an investigation conducted in The Detroit Edison Company laboratory on notch-toughness testing. The specific problem studied was to determine the influence of the uniformity of test results and on the magnitude of average notch-toughness values, both at room temperature and at 925 F, of three variables, namely, (a) the type of specimen, (b) the heat of steel, and (c) the condition of the steel. In connection with this testing program, a survey of the literature was conducted and inquiries were directed to various authorities regarding their experience with this problem. The authors conclude that when the significance of notched-bar testing of carbon-molybdenum pipe material is fully understood, it may be possible to select material of the best quality for high-temperature service of the basis of notched-bar tests.