Formation of defects during Zn diffusion into undoped and semi‐insulating Fe‐doped InP single crystals at 700 °C was observed by transmission electron microscopy for various diffusion conditions. Agglomerates of predominantly perfect interstitial‐type dislocation loops, dislocations, and small indium precipitates inside voids are observed in the Zn‐diffused crystal region. In addition, large planar arrays of precipitates are formed by climbing dislocations. From these observations it is concluded that the incorporation of Zn on In sublattice sites creates a supersaturation of In self‐interstitials which is relieved by dislocation loop formation leading to a supersaturation of P vacancies and void formation. ©1995 American Institute of Physics.