The phenomenon of slow amorphization during a constant-temperature and constant-pressure thin-film reaction is explained by a kinetic model emphasizing the rate of transition. We assume that the reaction obeys a maximum time-dependent rather than time-independent negative free-energy change. The product persists in the metastable state due to a high activation barrier to later transition. An amorphous Rh-Si alloy formed by thermally reacting a crystalline Rh thin film and single-crystal Si is reported. © 1991 The American Physical Society.