The redistribution of impurities in SOS (silicon‐on‐sapphire) ‐type structures during thermal oxidation is formulated and solved numerically for the nonlinear case when the diffusion coefficient depends on the impurity density. The method of lines is used to reduce the nonlinear boundary‐value problem to a stiff system of initial‐value ordinary differential equations, which is solved by Gear’s method. A computer program based on the numerical solution has been developed and applied to an illustrative example considered previously for the linear problem. The program was then applied to three realistic examples pertaining to the redistribution of ion‐implanted boron impurities in SOS‐type structures during thermal oxidation.