The experimental results for chemical and isoconcentration diffusion of Zn in AlSb, GaAs, and GaP have been interpreted using Longini's interstitial/substitutional model. Satisfactory agreement is found with the isoconcentration data if the substitutional Zn is a fully ionized monovalent acceptor and the interstitial is a monovalent donor. The characteristic features of chemical diffusion are explained in terms of a non‐equilibrium situation dominated by the rate of supply of vacancies on the group III sub‐lattice and the flux of interstitial Zn donors. Diffusion‐induced dislocations are assumed to play a major role in the supply of vacancies. Satisfactory qualitative agreement with the chemical diffusion data is found. Copyright © 1969 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim