The crystallization of sputter-deposited Si/Al amorphous alloys was examined by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). In-situ high-resolution TEM reveals the existence of an Al layer between the amorphous matrix and the growing crystalline phase. The activation energy for the growth is about 1.2eV, roughly corresponding to the activation energy of Si diffusion in Al. These two observations support the view that a crystallization mechanism, in which an Al buffer layer provides the shortest reaction path, is responsible for the reaction. The product microstructure exhibits secondary crystallization at a higher temperature.