The solid-state reaction of Mo/Si multilayer thin films produced by the r.f. magnetron sputtering technique was examined using differential scanning calorimetry (DSC) and X-ray diffraction and was explained by the concepts of effective driving force and effective heat of formation. In constant scanning-rate DSC, there were two exothermic peaks representing the formation of h-MoSi2 and t-MoSi2, respectively. The activation energy for the formation of h-MoSi2 was 1.5 eV, and that of t-MoSi2 was 7.8 eV. Nucleation was the rate-controlling mechanism for each silicide formation. The amorphous phase was not formed in the Mo/Si system as predicted by the concept of effective driving force. h-MoSi2, the first crystalline phase, was considered to have lower interfacial free energy than t-MoSi2, and by increasing the temperature, it was transformed into more stable t-MoSi2.