Reactive annealing of Cu-In-Se thin film stacks is an attractive technique to produce large area CIS absorbers for photovoltaic applications. In order to get detailed information on the mechanisms of CIS formation Thin Film Calorimetry has been used for in-situ monitoring the reaction pathway during heating. In a first step, the behaviour of binary In-Se and Cu-Se thin films is summarised as a prerequisite for the interpretation of the calorimetry spectra of ternary layers. It is shown in a second step that the information obtained exsitu by X-ray diffraction deviates from the results of the in-situ technique with respect to rapid re-transformation of Cu-selenides during cooling. The use of these complementary methods leads to a model describing qualitatively the reaction pathway of CIS-formation for a constant heating rate. In the temperature range above 380°C CIS growth proceeds by the reaction of Cu2-xSe and InSe in presence of a peritectic Se-rich liquid. This mechanism suggests an analogy with models for CIS growth during physical vapour deposition.