Disorder effects in CdSxSe1-x alloy thin films grown on Si(I 1 1) substrates are identified by X-ray diffraction. These effects are correlated with the localization of excitons observed in photoluminescence. Furthermore, X-ray diffraction patterns collected with a large area detector allowed us to point out structural disorder effects which cannot be observed by conventional 0-20 diffractometers. In the microdiffraction patterns, the broadening along the Debye rings reveals that the films are not perfectly epitaxially grown. Moreover, the lack of some reflections expected in the microdiffraction spectra indicates that further disorder effects are present. It is shown that these effects are not related to alloying, (C) 2002 Elsevier Science B.V. All rights reserved.