Abstract Diffusion intermixing in PbTe–PbSe epitaxial superlattice was investigated with help of the double-crystal X-ray diffractometry. Diffusion coefficients were defined for different annealing temperatures from the regular fading of satellites intensities on the X-ray diffraction patterns in the course of annealing. Theoretical grounds and computational treatment of experimental data are presented. Diffraction patterns have been simulated by numerical solution of the Takagi–Taupin equation and calculated curves were fitted to experimental ones accounting the superlattice profile smoothing due to diffusion. Diffusion coefficients determined by both procedures are in a good agreement.