Using Rutherford backscattering (R-BS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), the sulphurization of single-phase Cu-11 In-9 precursors to be employed as light absorbing CuInS2 (CIS) layers in CIS-CdS heterojunction thin-film solar cells has been investigated. The Cu-11 In-9 precursor films were produced by DC-sputtering from a single-phase Cu I I Ing target. The sulphurization at 5 00 or 3 00 degrees C was performed by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 degrees C, up to 50% of the indium initially present in the precursor is lost. We relate the In-loss to the volatile In2S compound, the formation of which is favoured by the phase transition of Cu11In9 to Cu16In9 at 307 degrees C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 degrees C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuSx surface phase. (C) 2006 Elsevier B.V. All rights reserved.