Quartz undergoes very significant luminescence sensitivity changes after high temperature annealing (0-1200 degrees C), with particular enhancement occurring between the phase transition temperatures 573 and 870 degrees C. In order to understand why this occurs, high frequency electron paramagnetic resonance (EPR), operating at 90 GHz, has been used to monitor the structure and population of defects in natural sedimentary quartz, following annealing and gamma-irradiation. The results are compared with the optically stimulated luminescence (OSL) data of the same samples. It is shown that: (i) the structure and population of the dominant [AlO4](0) recombination centres are largely unaffected by the annealing process; (ii) the oxygen vacancy E' centres are destroyed when annealed at temperatures between the phase transitions and; (iii) the numbers of both [TiO4/H+](0) and [TiO4/Li+](0) donors increase between 400 and 700 degrees C. Photo-EPR spectra are presented, providing evidence that both the Ti associated donors and Al accepters are directly involved in the OSL process. The heat-induced changes in the population of these EPR defects is mirrored in part by the change in the luminescence sensitivity of several OSL components. Evidence is also presented suggesting that E' may act as non-radiative centres competing in the OSL process.