The formation of CuInSe2-CuInS2 alloy films from chalcogenisation of different precursors was investigated by in-situ energy dispersive X-ray diffraction (EDXRD). A sequential synthesis procedure was used. Copper and indium (Cu/In = 1.8) were sputtered on molybdenum coated soda-lime glass and selenium was introduced as a layer of elemental selenium or as a In2Se3 layer. Such prepared precursor films were then sulfurized in elemental sulfur vapor. The effects of the selenium precursors and the influence of sulfurization conditions on the resulting absorber films composition and properties were investigated. It is shown that from different precursors, chalcopyrite phases are formed via significantly different formation pathways which are investigated in detail.