We present results of in-situ X-ray diffraction experiments on the formation of CuInS2 thin film solar cell absorbers. The experiments have been performed while annealing Cu-In-S stacked elemental layer precursors produced by sputtering and thermal evaporation to investigate the crystallisation process of the chalcopyrite CuInS2. Rietveld refinement has been performed to obtain the quantitative phase evolution of crystalline phases while annealing. The annealing process is characterised by a rapid sulfurisation of the initially present intermetallic alloy Cu11In9 forming the sulfide phases CuS, Dg-Cu2-xS, InS and/or CuIn5S8. The chalcopyrite CuInS2 crystallises at elevated sample temperatures by the consumption of these sulfide phases as educts. Three different chalcopyrite formation reactions have been identified by an analysis of the quantitative phase evolution. A comparison to earlier investigations on the formation of CuInSe2 from Cu-In-Se precursors is presented to show similarities and differences of sulfurisation and selenisation processes. The chalcopyrite forms from chalcogenide educts in both cases. However, distinct differences concerning the chalcogenisation kinetics of sulfur and selenium containing Cu-In precursors have been revealed. The chalcogenisation of the intermetallic alloy phase Culling proceeds extremely fast for Cu-In-S precursors as compared to Cu-In-Se samples. (C) 2008 Elsevier B.V. All lights reserved.