The CuInS2 thin film formation from a Cu/In precursor stack in the presence of elemental sulfur using a rapid thermal process under Cu-poor conditions has been studied. The process has been aborted at appropriate stages and the corresponding samples were investigated by XRD, Raman spectroscopy and SEM. The sulfurisation starts from elemental Cu and CuIn2. Elemental In and the binary phases Cu11In9 and Cu7In3 appear as intermediate phases. At the end of the sulfurisation the sample contains the ternary phases CuInS2 and CuIn5S8. CuS and β-In2S3 are detected by Raman spectroscopy at the sample surface and at distinct stages of the sulfurisation only. A difference in CuInS2 crystal quality is observed between the surface and the bottom of the samples.