Pyrolytic graphite is frequently used as a post diffraction monochromator with powder diffractometers of the Bragg-Brentano type. Curved graphite monochromators yield high intensities and suppress efficiently fluorescent radiation except from the element which is the same as the target element of the X-ray tube. They are not, however, able to separate the Kα1 line from the Kα2 component with the X-ray targets normally used in powder diffraction. Crystals with smaller mosaic spread than graphite are needed for selecting a pure Kα1 line. Commercially available crystals which are suitable for this purpose are asymmetrically cut curved quartz and germanium monochromators of the Johansson type. Experiments have been carried out in this laboratory with two quartz monochromators and one germanium monochromator.