[en] X-ray reflectivity studies of a synthetic 200 layer W₂₅Si₇₅ crystal on a Si wafer with a d-spacing of 25 A yielded the following results: At a fixed wavelength of 1.54 A the reflectivity for grazing incidence below a critical angle of 5.1 mrad exceeds 70%. The peak reflectivity of the first order Bragg reflection at the Bragg angle of 31 mrad is 78% of the totally reflected beam, and the relative band width of the peak is 2.1%. Both these numbers are in good agreement with the simplest form of dynamical scattering theory as outlined in the text. The reflectivity of higher harmonics is less than a few per cent, with the fundamental set for reflection at 1.54 A. Crystals of this quality are very useful optical elements in synchrotron radiation instrumentation, in particular when adequate cooling methods are fully developed. (orig.) With 4 figs., 4 refs