We have investigated reactive phase formation in magnetron sputter-deposited Ni/Al multilayer films with a 1:3 molar ratio and various periodicities ranging from 320 nm to a codeposited film with an effective periodicity of zero. The films were studied by x-ray diffraction, differential scanning calorimetry, electrical resistance measurements, and transmission electron microscopy. We find that a reaction which results in the formation of an amorphous phase has taken place during the multilayer deposition process. This reaction substantially reduces the driving force for subsequent reactions and explains why nucleation kinetics become important for these reactions. The mode of transformation for a film with 10 nm periodicity was investigated, in detail, by applying the Johnson-Mehl-Avrami analysis to data obtained from isothermal and constant heating rate differential scanning calorimetry, in combination with electron microscopy studies of the transformation microstructure.