The preferential sputtering of a binary alloy is considered using diffusion theory. It is shown that this leads to non-linear-terms in both the bulk diffusion equation and the boundary conditions. The steady-state solution is investigated in detail, and the results indicate that some of the non-linearities are negligible, while others cannot be omitted without appreciable errors. The surface depletion of the more volatile component is much smaller than earlier diffusion studies had suggested.