Energy dispersive X-ray diffraction (EDXRD) has been used to observe in situ the formation of Cu(In,Ga)S-2 (CIGS) films during reactive magnetron co-sputtering. By simultaneous sputtering from a CuGa and an In target in an Ar:H2S atmosphere the CIGS films, which can be used as absorber layers for thin film solar cells, can be grown in a single step without any post-treatments. By carefully analyzing the in situ experiments, several effects have been found which should be considered during designing the recipe for an absorber deposition process in order to achieve CIGS films with the desired stoichiometry and good crystallinity. For instance substrate temperatures below 380 degrees C should be used in order to achieve a sufficient condensation of In atoms which is needed to avoid the formation of intermetallic Cu-Ga phases. Since, on the other hand, high substrate temperatures are necessary in order to grow large crystallites, a process which starts at a low temperature and has temperature ramp up later on is possible because In which is bound in the chalcopyrite phase evaporates only for much higher temperatures. These results demonstrate the opportunities of EDXRD for the real-time analysis of the growth process of thin films which can provide information not obtainable by any ex situ characterization method. (C) 2013 Elsevier B.V. All rights reserved