The diffusion of zinc into GaAs and (GaAl)As was studied using a high resolution microsectioning technique. Diffusions were performed into epitaxially grown thin layers comparable to those used in injection laser structures; several boat grown GaAs were also studied. The diffusions were done at a temperature commonly used in device processing; a three-phase diffusant was used to preclude sample dissociation and damage. A diffusion coefficient of Zn in GaAs of 1.4×10-11 cm2 s -1 was obtained, and this is in good agreement with the extrapolation of the values obtained at higher temperatures. A marked difference in the diffusion profiles of GaAs and (GaAl)As was observed.