The lateral composition modulation in ( InAs )_n/( AlAs )_m short-period superlattices was studied by means of synchrotron x-ray diffraction. By choosing specific diffraction vectors having a large component closely parallel to the modulation direction, we are able to observe a number of lateral satellite peaks around the zero-order short-period superlattice peak. A model, incorporating both composition and strain, is used to simulate the intensities of these satellites. Our results provide a quantitative fit and permit the evaluation of the composition amplitude. © 2001 American Institute of Physics.