On the basis of our experimental results on the influence of radiation of different kinds on a number of crystals with ionic-covalent bonds and analysis of the well-known data, a criterion of radiation-induced structure transformation of a high-temperature type is proposed. It is the only type of complete structure change in irradiated nonmetallic material which is not caused by decomposition or amorphisation and following recrystallisation of initial compounds. It is also found that in a number of materials structure changes are caused by the radiation-induced point defects and not by the formation of high-temperature phase nuclei.