The kinetics of intermediate phase formation in (20 nm) Cu/(80 nm) Mg multilayers is analyzed using differential scanning calorimetry (DSC) and in situ x-ray diffraction. The as-prepared Mg layers are polycrystalline with large grain sizes and a columnar structure which gives a preferred orientation along the (0002) direction. On the contrary, the Cu films are nanocrystalline with average nanocrystal populations of different sizes. The continuos scan DSC traces show three main exothermic peaks. The first two reactions are significantly overlapped. We find both processes are related to nucleation and growth of the Mg2Cu along the interface and present an explanation based on the different roughness of the nc-Cu on Mg and the Mg on nc-Cu interfaces. The third exothermic reaction is due to the growth of the Mg2Cu perpendicular to the original interface. A kinetic model is developed which yields calculated calorimetric traces in very good agreement with the experimental data. The model includes two independent nucleation & growth mechanisms. A soft-diffusion control of the one-dimensional thickening of the Mg2Cu product phase is included to account for the gradual decrease associated to the end of the third calorimetric peak. © 2003 American Institute of Physics.