Abstract: A plane-grating monochromator is described which is particularly well suited for wavelengths in the range from approximately 1 to 15 nanometers and contains as its primary optical elements a diffraction grating (16) and an ellipsoid mirror (20). If both the entry aperture and the exit aperture are intended to be stationary, then a pivotable mirror (14) preceding them is provided. The present plane-grating monochromator is distinguished by an accurate image, virtually free of distortion, and correspondingly high resolution as well as by simple optics; it includes only two or three optical elements, and a kinematically simple adjusting mechanism for the optical elements suffices for adjusting the wavelength.