Phase transformations of GaxSy -(Cu,In) thin film stacks annealed in sulphur vapour were investigated by in-situ energy dispersive X-ray diffraction (EDXRD) in order to monitor the Ga incorporation into the resulting quaternary Cu(In,Ga)S-2 alloy. The partial replacement of In by Ga in the chalcopyrite structure widens the band gap of the material. Thereby the open circuit voltage of corresponding devices can be enhanced. In the present study Ga was supplied by an amorphous or crystallized (GaxSy)-layer, deposited by thermal evaporation of Ga2S3 before sputtering elemental Cu and In precursor layers. The sulphurisation was carried out in a specially designed vacuum chamber which can be attached to the synchrotron beamline F3 at Hasylab (Hamburg). The evolution of the different phases during sulphurisation of the GaxSy - (Cu,In) precursors, as observed via EDXRD, indicated the almost simultaneous occurrence of two stacked chalcopyrite phases which are Ga-rich and In-rich. For longer annealing times the EDXRD-spectra show the appearance of a third, intermediate Cu(In,Ga)S-2 phase. Monitoring the peak position during the annealing stage allows evaluating the lattice parameters as a function of annealing time. Thereby the In-Ga interdiffusion between the different chalcopyrite phases can be investigated. (C) 2006 Elsevier B.V. All rights reserved.