When an intrinsic defect (e.g., vacancy or self-interstitial) mediates self-diffusion, the activation enthalpy is equal to the sum of the defect's formation and migration enthalpies. If the migration enthalpy is small and the migration entropy is large, however, at some critical temperature the saddle point becomes the stable configuration, whereby a totally different path may become dominant. It is, therefore, possible that paths with higher activation enthalphies may dominate at high temperatures, giving rise to a curvature in the Arrhenius plot of the diffusion coefficient. These effects are very likely to occur in crystalline Si and may account for the observed large entropy in high-temperature self-diffusion. © 1987 The American Physical Society.