A high resolution x‐ray diffractometer is described in which a compact four‐crystal monochromator produces a highly parallel and monochromatic incident beam of tuneable wavelength (divergence Δθ=5″, wavelength band Δλ/λ=2.3×10− 5, intensity 3×104 photons/s mm2). The instrument is very suitable for the nondestructive measurement of concentration depth profiles produced in semiconductorsingle crystals by epitaxy,diffusion, or implantation. The equipment is useful for measuring x‐ray rocking curves of any lattice plane of any material and also lattice constants can be determined on absolute scale using the Bond procedure. A special solution has been derived from the dynamical theory of x‐ray diffraction, which gives the reflectivity of an absorbing layer as a function of layer thickness. This new expression takes the effects of absorption and extinction into account and has been used to determine the layer thickness of epitaxial layers grown on [001] oriented InP.