In III-V semiconductors a common mode of vacancy migration is by nearest-neighbour hopping. In the presence of Zn diffusion this mode is greatly enhanced. A consequence is the production of antisite defects, which usually become antistructure pairs as the vacancy returns to its original sublattice. Many important deep-level defects involve vacancies during their creation. One should expect to find neutral antistructure pairs in the vicinity of Zn-O luminescence centres, EL2 defects, etc. Much of the mystery of these centres can be attributed to the dipolar interaction of these antistructure pairs with the nominal defects.