The kinetics of Si crystallization during isothermal annealing of Au/polycrystalline-Si bilayers were investigated via scanning electron microscopy and in situ sheet-resistance measurements. Large crystals of Si grow within the Au layer in the shape of two-dimensional plates of thickness equal to that of the Au layer. The Si crystals grow at an average rate which follows a t2 time dependence with an Arrhenius temperature dependence of e/kTE, where E2.4 eV. The overall transformation rate is a product of the two separable processes: the first determines the number of crystals and the second the growth rate of the crystals. At the beginning of the reaction the number of crystals, N, per unit area is fixed, remains constant throughout the anneal, and has an Arrhenius temperature dependence of NeN-E/kT where EN1.0 eV. The crystals grow at a constant growth rate and have an Arrhenius temperature dependence of vev-E/kT with Ev1.9 eV. Final analysis of the overall transformation relates the activation energy of the total process to the activation energy of adding or removing an atom from the Si surface (Ev) and the diffusion of Si in Au (ED): E=3/2Ev- 1) / 2 ED.. AE © 1990 The American Physical Society.