Diffusion, at constant surface concentration, with coefficient varying to the positiventhpower is considered. It is shown that the diffusion equation for these cases has a unique solution whose leading term is concentration ∝Z1/n, where z = 1−(x/x+),xbeing the distance from the surface andx+the distance at which the concentration just reaches zero. Numerical solutions can be obtained by standard methods of solution; explicit expressions are given forn= 1, 2, and 3. It is also shown that plots of log(concentration) versus logz give rapid reliable estimates ofnfrom usual diffusion profiles. These methods are applied to previously analyzed data for diffusion of Zn in GaAs at 1000 °C. This reanalysis suggests that (i) the diffusion coefficient for Zn in GaAs is accurately given by the ``interstitial‐substitional'' mechanism at thesurfaceof GaAs, (ii) for surface concentrations ofZn⪞5×1019 cm−3, the diffusion is influenced by a limited flux of gallium vacancies from the surface, and (iii) the diffusion coefficient for gallium vacancies is roughly 5×10−9cm2/sec at 1000°C. It is suggested that ``double'' profiles, often found for surface Zn concentration⪞1020 cm−3, are due to dual vacancy sources: the free surface and a dislocation net occurring at positions of large Zn gradient.