Formation of defects during Zn diffusion into undoped and Fe-doped InP single crystals at 700°C has been observed by transmission electron microscopy for various diffusion conditions. The observations are correlated with Zn concentration profiles obtained by electron microprobe measurements and secondary-ion mass spectrometry. The results allow the conclusion that indiffusing interstitial Zn can occupy In sublattice sites via a kick-out reaction. Under appropriate diffusion conditions supersaturations of In self-interstitial atoms result leading to defect formation. Observations in Fe-doped InP suggest that Zn also replaces Fe on In sublattice sites leading to redistribution and to precipitation of Fe.