The problem of diffusion coefficients with fixed surface composition and variable diffusion coefficients is considered in the present paper. The rate of solute uptake or loss is given using the same equations as for constant diffusion coefficients taking the diffusion coefficient equal toD=approximately the first moment of the diffusion coefficient about the surface composition. When a new phase forms on the surface of the diffusion couple and it has a small composition range the rate of movement of the phase boundary is given by the same equations as for constant diffusion coefficients, but usingD¯, the weighted mean diffusion coefficient. As the composition range of the phase increases, the diffusion coefficient to be used deviates towards the value of the diffusion coefficient at the surface. The results are illustrated with calculated single‐and two‐phase composition profiles for carburization of a steel specimen.