Molecular beam epitaxy CuInS2 layers were grown on Si single crystals of different orientation. The stoichiometry was varied between 0.5 and 1.5 for the nominal Cu/In ratio and the film thickness was limited to 150 nm. Composition analysis was done by in situ photoelectron spectroscopy (PES), Rutherford backscattering (RBS) and X-ray diffraction (XRD). PES was used to determine the surface composition, RBS monitored the volume contribution and XRD probed phase formation. It was found that the surface composition differed significantly from that of the volume: instead of the expected variation along the Cu2S-In2S3 binary, the actual composition varied along the Cu2S-InS line at the surface (S-deficiency). The role of sulfur incorporation into the growing film is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.