A theoretical model called the ‘‘multibody model’’ is developed for the composition dependence of the activation energy. The model that is based on the diffusion required of the recrystallization for the solid phase epitaxy does not involve any adjustable parameters and is shown to represent experimental data satisfactorily. For the Si1−xGex alloys that are of diamond structure, the most logical choice is the ‘‘five-body model’’ involving five atoms. The model is equally applicable to any other binary alloy.