When a single crystal is used in an x-ray spectrometer to display the continuous spectrum of an x-ray tube by Bragg diffraction, absorption edges characteristic of the atomic species in the crystal may be observed. The change in diffracted power across an edge is fundamentally due to a change in the atomic scattering, especially the imaginary part of the scattering, of the species whose edge it is. This produces a sharp anomaly in both the structure factor and the absorption factor of the reflection being used. The explicit effect of the absorption is a function of the perfection of the crystal face; the change in the structure factor depends uniquely on (hkl) for noncentrosymmetric crystals. Thus, the absorption edges may be used as a tool to study perfection of crystal faces and polarity of polar axes. The K edges of Ga and As seen in reflection from (111) and (1̄1̄1̄) faces of perfect and mosaic crystals of GaAs are discussed both theoretically and experimentally in some detail. The linear absorption coefficient of GaAs and Ge have been measured in the course of the work. © 1962 The American Institute of Physics.