Thermal instabilities due to the recombination of the Frenkel defects have been theoretically studied. The release of the energy stored in the Frenkel pairs leads to a temperature increase which, in its turn, accelerates the process of thermally activated recombination. For sufficiently small heat conductivity and heat transfer to the cryostat this “feed-back” process can lead to a jump-like temperature increase. The criteria for the occurrence of such thermal instabilities have been obtained for the case of the annealing the sample after irradiation and for the case of high defect production rate during irradiation. The criteria depend essentially on the value of the activation energy,W, for the process of recombination; sample size, a; heat transfer coefficient,h;and the concentration of the recombining Frenkel defects,f.