Sulphurisation of CuInSe2 solar cell absorber materials is of particular interest for band gap engineering by means of compositional grading. We studied the chalcopyrite formation process of Cu-In-S-Se stacks with varying Se to S ratio from 100% Se (0% S) down to 0% Se (100% 5). Differential calorimetry (DSC) was applied as a fundamental characterization tool to track the crystal phase formation process. To verify the evolution of various crystallographic phases, DSC measurements were supplemented by temperature dependent in-situ X-ray diffraction (XRD) experiments of equivalent thin film compositions. For a S content of 25% the system is governed by sulphurisation of Cu and In, with the ternary phase CuIn5(S,Se)(8) and the binary phase Cu-2(S,Se) playing major roles in the chalcopyrite formation process. It is found that the chalcopyrite formation moves to higher temperatures if the S content is increased. (C) 2010 Elsevier B.V. All rights reserved.