An x‐ray diffraction study of the growth of Pb1−xMnxTe/PbTe (x=0.022–0.037) multi‐quantum well (MQW) structures of period lengths 205–950 A˚ is presented. In order to obtain a reliable Mn diffusion coefficient at the growth temperatureT=270 °C, computed diffusion profiles are used as input for the calculation of diffractograms using kinematical diffraction theory. The satellites of the symmetric (222) Bragg reflections are compared to measurements. The analysis is based on a numerical solution of the diffusion equation which considers explicitly the effect that the boundary at the surface of the growing epitaxial layer moves during growth. Thus for the as‐grown MQWs Mn interdiffusion is considered to occur already during the growth process. In addition, the Mn diffusion profiles of one sample which is isothermally annealed 4, 16, and 64 min atT=270 °C, are simulated using fixed boundary conditions. From comparison to both boundary conditions, the Mn diffusion coefficient of 8×10−17cm2/s atT=270 °C is determined within an accuracy of a factor two. The results are well adapted to our special growth conditions, but the numerical model is applicable in generality. It uses only two essential input parameters: the diffusion coefficient and the growth rate.