Abstract The effects of ageing on the thermoluminescence of LiF(TLD-100) crytals have been investigated by computerized glow curve analysis. After an annealing of 1 h at 400°C and quick cooling the crystals were aged at 80°C for 0, 6, 20, 60, 200, 800 and 1200 min and readout at 6°C s −1 or 0.24°C s −1. The intensities of peaks 2, 3 and 4 decrease with increasing ageing time. Peak 5 initially increases (up to 20%) due to clustering of dipoles and decreases at longer ageing times due to precipitation. Dramatic changes in the values of the activation energies of peaks 2 (from 1.35 to 0.72 eV), 3 (from 1.44 to 1.17 eV) and 4 (from 1.55 to 2.32 eV), have been observed for ageing times longer than 20 min and a readout heating rate of 6°C s −1. These changes are smoothed out at when the readoutheating rate is slow. The strong changes of E values indicate a trap modification process which may be caused by precipitation of defect clusters. The relevance for practical dosimetry is discussed.