CuInSe2 thin films were prepared by sequential evaporation of the constituent elements onto Mo-coated glass substrates and the layers were subsequently selenized in a closed graphite box at 400 degrees C. The films were characterised by X-ray diffraction (XRD) and in situ, realtime Rutherford backscattering spectrometry (RBS) analysis to monitor the progress of the reaction as the elemental layers interdiffused. For the Cu/In/Se structure, XRD performed after various anneals revealed that the Cu-In layer mixed with the Se layer to form a number of binary phases. For Cu/Se/In it was found that the Cu and Se had intermixed at room temperature. In both the structures investigated the Cu-Se and In-Se binary phases formed below 200 degrees C and upon annealing above this temperature interdiffused to form chalcopyrite CuInSe2.