An approximate theory of laser-induced diffusion in Si is presented. Starting with the concept of molecular motion in liquids, the self-diffusion coefficient of liquid Si at the melting temperature is calculated. By introducing the temperature dependence of diffusivity in the melt, the maximum diffusivity as a function of the laser-power density, the pulse duration, and the As implant dose is derived. Approximate expressions for the depth of melting and the time the Si surface remains molten yield an understanding of the implant dose and the laser-energy dependence of the diffusion depth. Finally, computer simulations of laser-induced diffusions are used to verify that As evaporation occurs to some extent during laser annealing.