LiF-TLD 100 exposed to ionising radiations and then suitably annealed shows an enhanced thermoluminescence response for the second exposure. Several models have been proposed to explain this phenomenon which has been termed 'radiation-induced sensitisation'. In this paper it is shown that the sensitisation is observed only in LiF and KCl doped with impurities having limited solid solubilities. It is proposed that the sensitisation occurs owing to the redistribution of the impurities during the thermal annealing of the exposed samples.