X-ray diffraction has been extended by simultaneously measuring inelastic signals excited internally by the X-ray standing wave field. As the inelastic X-ray standing wave signal we recorded the photoelectron current from an (AlAs)m(GaAs)n short-period superlattice on a GaAs(001) substrate. It shows clear modulations due to the GaAs(004) substrate reflection, the (AlAs)(GaAs)(004; 0) superlattice main reflection and the thickness fringes. The modulation at the (AlAs)(GaAs)(004; 0) reflection is related to the content of atoms in lattice plane positions. This is of technological interest, since the coherent fraction offers a new parameter for the characterization of the quality of short-period superlattices. The X-ray diffraction measurement in a wide angular range gives the complete set of satellites between two neighbouring substrate reflections. For some of the samples investigated, a non-coincidence of the (AlAs)(GaAs) satellite groups belonging to GaAs(002) and GaAs(004) is observed, which can be shown to be due to an incommensurate modulation of the superlattice. From the satellite intensities, the Fourier coefficients of the modulation can be determined by comparison with kinematical diffraction theory. This gives a description of the (AlAs)m(GaAs)n superlattice unit cell and its interface widths.