It has been observed that the thermoluminescence of heavily gamma-irradiated crystals of NaCl and KCl is inhibited up to temperatures higher than those corresponding to less irradiated crystals. In NaCl it is clearly seen that the vacancy-centre aggregation processes including the formation of the colloidal band are operative before thermoluminescence occurs. During the thermal bleaching of the colloidal band F-centres are produced which in turn are annihilated by interstitials. The temperature of the maxima of the glow peak agrees well with the temperature at which the radiation-induced hardening anneals out. As in the model proposed in a previous paper (see abstr. A16630 of 1972 and A8677 of 1973), it is concluded that the thermoluminescence in irradiated alkali halides is due to the recombination of mobile interstitials with vacancy centres; at some stage in this process light is emitted.