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Temperature measurement and control are key issues in many technologically important areas of semiconductor processing, including rapid thermal annealing, molecular‐beam epitaxy, and plasma processing. As a result, the development of accurate techniques for the characterization of thermal cycles is a problem of great practical interest. In this study, the total emissivity of specimens of GaAs was determined using a technique which combines isothermal electron‐beam heating with a temperature measurement method which exploits the temperature dependence of the band gap of GaAs. Emission spectra from the GaAs specimens were recorded for a range of heating power densities. These spectra display a maximum near the semiconductor absorption edge, because the blackbody radiation rises with increasing wavelength but the spectral emissivity decreases rapidly once the photon energy falls below the band gap. The temperature was determined by fitting the Planck radiation function to the high‐energy side of the maximum. This allowed a self‐consistent determination of the temperature dependence of the position of the absorption edge. The results were used to calibrate a second set of experiments in which a corresponding set of reflection spectra were recorded. The reflection spectra exhibit a large change in reflectivity at the absorption edge, because light starts being reflected from the back surface of the wafer as well as from the front when it becomes transparent. The reflection spectra provide a convenient method of temperature measurement because the intensity of the reflected light can be much larger than that of the emitted light. The large signal levels should permit temperature measurements to be made with good spatial and temporal resolution, even for samples at low temperatures. The temperature measurements can be performed with a precision of about 2 °C, and in this study the accuracy was ±5 °C. As a re- sult of these measurements the total hemispherical emissivity of the GaAs specimens was determined over the temperature range from 350 to 630 °C. The values for the total emissivity of GaAs reported in this paper are believed to be the first direct, experimental measurements of this quantity. It was found that the total hemispherical emissivity of 485‐μm‐thick specimens rises from 0.08 at 350 °C to 0.28 at 630 °C. The electron‐beam heating method used in this work presents distinct advantages for this type of measurement because there is little background light in the apparatus, the energy coupling to the specimen is not affected by its optical properties, and the surroundings remain cool. The power input can be accurately determined from the electron‐beam energy and current and the scan area, and hence the total emissivity can be determined in a straightforward manner.