Solid state devices based on copper indium diselenide, (CIS), have produced world record efficiencies for polycrystalline photovoltaic solar cells. in order to understand thin film phase transformations that are relevant to the production of CIS, multilayer thin films of copper and indium have been analyzed. Using differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy, the reaction pathways for equal atomic concentration, copper rich, and indium rich films were established. Indium was found to react during deposition, so that the as-deposited films consisted of CuIn2 and copper. During constant heat rate experiments, the excess concentration of copper within the film provided the driving force in the formation of phases richer in copper. The Cu11In9 phase was observed to form from CuIn2 and Cu at 130 degrees C, and Cu11In9 formed from Cu11In9 and Cu at 310 degrees C. In the indium rich samples, a given phase was found to be stable until melting. In firms with an overall indium concentration exceeding the concentration of Cu11In9, the excess indium was shown to move primarily to the film surface, and in indium rich films, to grain boundaries between the Cu11In9 phase.