The microstructure induced by the Zn diffusion at 1170 K into doped and undoped semi-insulating GaAs single crystals was characterized for various diffusion times t < 1740 min by analytical electron microscopy. The results were compared with Zn concentration profiles obtained by spreading resistance measurements (SRM) on the same samples. At the diffusion front the formation of prismatic interstitial dislocation loops, dislocation networks, and of cavities partly filled with Ga was observed. Closer to the surface facetted voids and, for the undoped samples, vacancy-type dislocation loops formed. The near surface region of highest Zn-concentration showed a high density of Zn-rich precipitates. A model is presented which accounts .for these observations. It is based on fast interstitial Zn diffusion and the kick-out mechanism for interstitial-substituional exchange.