CuInS2 layers for solar cells have been sequentially prepared by a rapid thermal process using sulfurisation in molecular S. Samples taken from the process at different sulfurisation times have been analysed by Raman scattering, AES and TEM in combination with EDX in order to investigate the sulfurisation process. After deposition of the Cu and In precursors, a bilayer structure is formed: Cu at the lower part and a Cu-In alloy at the upper. The results indicated that the sulfurisation starts at the surface by directly converting the Cu-In alloy into CuInS2. During further processing, the sulfurisation continues at the already formed CuInS2-(Cu-In) alloy and CuS is formed at the surface of the sample. After only 120 s of processing, complete sulfurisation is achieved, leading to a bilayer structure CuS/CuInS2. The electrical performance of solar cells made from the absorber layers show no diode behaviour as long as binary Cu-ln phases are present acid, for the latter sample, a record 11.4% efficiency has been achieved.