The study of the physical properties of the superlattices (Ga 1−xAlxAs)n1(GaAs)n2/GaAs(001) requires the knowledge of their structural parameters n1, n2 and x and of their crystalline state. X‐ray diffraction enables the existence of such artificial superstructures, obtained by molecular‐beam epitaxy, to be revealed and their characteristic parameters to be determined. Two experimental techniques are used: a standard powder goniometer and a double‐crystal diffractometer. The diffraction diagrams directly yield the superperiod and the average Al concentration in the superlattice. The value of x is determined by refinement between observed and calculated structure factors. The results are even more accurate when the number of observed satellite peaks for a given periodicity is greater; this number depends at the same time on the Al composition x, the n1/n2 ratio, the periodicity and its dispersion and the shape of the interfaces. This method is illustrated by a few examples with superlattice periods varying from 51 to 397 Å. The consequence on the X‐ray diagrams of various defects (dispersion in n1 and n2, superperiod gradient, Al diffusion) is discussed.