A high‐resolution optical study was carried out on GaAs crystals grown by horizontal Bridgman and liquid‐encapsulated‐Czochralski methods. An excellent correlation was found between the intensity of the 1.039‐eV no‐phonon line and the characteristic absorption of EL2, the major deep donor level in GaAs. A correlation was also found between the characteristic optical absorption of EL2 and its concentration as determined by junction capacitance measurements. The presence of EL0, another midgap level contained in heavily oxygen‐doped crystals at concentrations always less than those of EL2, had no effect on the optical spectra, but altered the capacitance measurements. Accordingly, an accurate calibration for the determination of EL2 by optical absorption was obtained from capacitance measurements on crystals containing only EL2; in this way the uncertainties introduced by other midgap levels were eliminated.