Rapid thermal processing (RTP) under high sulfur partial pressure of copper rich Cu-In alloy thin films is investigated using in situ energy dispersive X-ray diffraction (EDXRD). Cu-In precursors are sulfurized at S vapor pressures in the 1 mbar range. Diffraction of white synchrotron light and recording of EDXRD spectra every 10 s at the EDDI beamline of the BESSY facility is used to monitor in situ the solid phases during the sulfurization and the subsequent cool-down. Ternary CulnS(2) forms via the binary InS and CuS and the ternary CuIn5S8 phases. The concentration of copper in the secondary Cu2-xS phase, which segregates on the surface of the CulnS(2), shows a strong dependence on the maximum sulfur pressure during the RTP-like process. (C) 2008 Elsevier B.V. All rights reserved.