Phase formation processes in coevaporated CuIn-films (precursors). and the formation of CuInS2 from these precursors by reactive annealing were investigated. Precursors were produced by simultaneous thermal evaporation of the elements onto neat glass substrates. The Cu-In precursors were sulfurized in a diluted H2S/H-2 atmosphere at various H2S concentrations. Subsequently, the formation of CuInS2 was studied after both incomplete and complete sulfurization. The films were characterized by means of Rutherford backscattering spectroscopy (RBS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The analysis of the XRD data was carried out by a RIETVELD type algorithm. It was found that next to composition and morphology of the precursors, the concentration of H2S in the reactive atmosphere is a significant parameter for the formation process and the microstructure of the resulting CuInS2 films.