AbstractThe diffraction of high‐energy characteristic x‐ray lines, which have relatively large intrinsic widths, by perfect single crystals, which have acceptance angles much less than these line widths, is discussed. Experiments using Bi Kα1, which has an equivalent FWHM of 0.0013°, and an asymmetrically cut Si (111) crystal, having acceptance angles of 0.0002° and 0.0004°, showed that only a small fraction of the total characteristic line was, in fact, diffracted by the