Defects, notably interstitial impurities, are responsible for degradation of resonator performance with time. In order to study the mobility of ionic impurities, we have measured thermionic currents as a function of temperature by the thermally stimulated depolarization (TSD) method. We have observed several current maxima; each peak is due to the displacement of an impurity family from a given site and is characterized by its activation energy. Making impedance measurements to complete these results, we find energies of about 1 eV. We have studied quartz crystals doped with a given impurity in order to identify the nature of the impurities responsible for some current peaks. This study confirms that the TSD method is very sensitive. ©1996 American Institute of Physics.