Thermal treatment of CaF2has a significant influence on the number and intensity of the peaks seen by thermo-luminescence. A combination of ion implants and anneal cycles leads to the conclusion that the 90°C glow peak is derived from a defect of a substitutional trivalent impurity (e.g. Ce+3) linked to an interstitial fluorine ion. Perturbations of this centre by other defects modify the centre and the glow peak temperature is raised to 110°C.