The indirect energy gap Eg of GaP in the temperature range from 0 to 900°K was determined from absorption measurements. Below 400°K, Eg was determined from absorption due to the creation of excitons at isolated nitrogen atoms substituted on P sites. This absorption was measured with wavelength-modulated exciting light. The intrinsic absorption edge was measured over the full temperature range by conventional transmission. Above 400°K, the gap was determined from a numerical fit of the intrinsic-edge absorption data to a theoretical expression using parameters obtained from low-temperature measurements, with Eg as the only adjustable parameter. The shapes of the experimental curves are in good agreement with the theoretical calculations. At 300°K, Eg is 2.261 eV. Light emission from diodes was studied from 300 to 900°K. The principal high-temperature emission line parallels the energy gap at about 45-meV lower energy, and is attributed to an exciton recombination. © 1968 The American Physical Society.