Precise relationships between the diffusion coefficientDand the zinc concentrationCare obtained from isoconcentration diffusion experiments performed at 900°C for zinc in GaAs and at 1000°C for zinc in GaP. It is found thatDvaries withCfrom a slightly less than cubic to a somewhat less than square dependence over the concentration range 1018to 1020cm−3.Possible mechanisms that result in concentration‐dependent diffusion are discussed. Under the conditions given above, it is concluded that diffusion occurs by an interstitial‐substitutional mechanism with the interstitial mode being dominant. The charge states of the various species involved in the diffusion are believed to be: (1) interstitial zinc atoms are doubly ionized donors; (2) substitutional zinc atoms are either neutral or singly ionized acceptors; and (3) gallium vacancies are neutral. A theoretical expression forDversusC, derived under these assumptions, fits the isoconcentration diffusion data over wide ranges ofDandCthrough the use of only one adjustable parameter.