Abstract The thermoluminescence light output at different stages of thermal bleaching and the corresponding changes in colour centres, have been investigated in the case of sodium chloride and lithium flouride crystals, coloured at room temperature by X-rays. It has been observed that, during the early stages of thermoluminescence, the F centers are partly transformed into other centres responsible for the long wavelength colour bands: thermal release of lattice vacancies in excess of the stable concentration at any temperature makes it possible for the lattice imperfections to be mobile at comparatively low temperatures. In the case of coloured sodium chloride, during the first stage of thermal bleaching upto about 100°C, the F band decreases in intensity giving thermoluminescence and also producing another band at 580 mμ. This band seems to be due to aggregates of F centres and is found to be responsible for the thermoluminescence emission at temperatures above 200°C. The thermal bleaching of coloured lithium flouride is found to occur roughly in three stages. The thermoluminescence light output given out by the coloured crystal when heated to 180°C is due to the partial destruction of F-, colloid-, R 1 and R 2 bands. In the region from 180°C to 340°C the largest light output is observed and it is mostly due to the bleaching of M and F centres. In the final stage of bleaching from 340°C to 380°C, a large amount of light has been given out due to destruction of the F band and colloid centres. The results of the present investigation suggest that the emission bands at 519 mμ and 416 mμ observed in the thermoluminescence of sodium chloride are due to radiative recombination of holes with the electrons trapped at F and aggregate centres respectively. The present measurements on lithium fluoride crystals indicate that four different colour centres are involved in the emission process; thus the variation in the spectral composition of the emission during thermoluminescence is likely to be due to the corresponding emission bands.