We analyze in this paper the effect of thermal treatment on structural properties of GaAs/InGaAs strained‐layer superlattices grown by molecular beam epitaxy. The superlattices are analyzed using double‐crystal x‐ray rocking curves. In order to model the satellites intensity variation as a function of the heat treatment time at a temperature of 850 °C, we have calculated the structure factors of the superlattices, taking into account both composition and lattice spacing modulation. The latter is found to be more influent in the calculation in this particular case. The deduced values of the diffusion coefficient, about 2×10−18cm2/s, is discussed and compared to those determined on GaAs/AlAs structures.