Diffusion redistribution of ion-implanted boron and phosphorus in silicon at 1000°C in nitrogen, dry-oxygen and wet steam ambients has been inverstigated both experimentally and by computer simulation. Measured diffusion profiles cannot be obtained if we solve the diffusion equation with a constant diffusivity. Supposing that the diffusivity D is a function of dopant concentration N we have obtained the dependences D(N) for considered cases of boron and phosphorus diffusion in silicon. For both impurities the diffusion in oxidizing atmosphera found to be enhanced. The enhancement is stronger for boron than for phosphorus.