Modern semiconductor processes, in which low temperatures, short times, and ion implant or oxidation‐induced interstitials may occur, raise the possibility that the mobile species may not be in equilibrium with their immobile parent substitutional dopants. The simplest possible example of nonequilibrium diffusion is analyzed and shown to occur in three stages. In the first, where the diffusion time is short compared with the lifetime of the mobile species, the dopant profile is an error function complement. The second stage, in which the diffusion time is long compared with the lifetime of the mobile species but short compared with that of the parent species, yields an exponential profile. The third stage, for long diffusion times, corresponds to the usual equilibrium assumption, and the measured profile is again a complementary error function.