We propose a model to explain the experimentally observed ion-beam mixing rates in reactive systems under nonoverlapping spike conditions. There are several technologically important metal-semiconductor systems belonging to this class. For comparison with model prediction, we have studied experimentally ion-beam mixing in Cu/Ge bilayer systems at various temperatures. The results show that the room-temperature mixing rate varies linearly with deposited energy density (Formula presented) and that the mixing is a diffusion controlled process proceeding via compound formation. Excellent agreement has been obtained between the experimental observations and theoretical values predicted from our proposed model while other current models are seen to be inadequate. The model also explains the mixing rates of the Ni/Si system reported in the literature. © 1996 The American Physical Society.