A detailed study has been made of the effect of cooling rate on (a) the thermoluminescent sensitivity of TLD-100 and Harshaw pure LiF, and (b) the Mg⁺⁺V associated pair concentration in TLD-100. The thermoluminescence of TLD-100 at the lower cooling rates appears to be strongly affected by precipitation of a second phase, whereas changes at higher cooling rates appear to be a consequence of Mg⁺⁺V clustering. In particular there is evidence to show that peaks II and III are probably associated with the same trapping centre and that this is the Mg⁺⁺V pair.