Electrical conductivity of &agr; quartz is ionic, the current being carried by alkali ions (M+) moving in channels parallel to thecaxis. A study was made of conductivity &sgr; of one natural and three synthetic crystals in order to better understand these ionic defects. ac measurements were made over a wide range of frequencies, and complex‐impedance analysis was used to separate out the bulk conductivity from electrode effects. The activation energyEfor the synthetic crystals was always close to 1.36 eV and the pre‐exponentialAwas in the range of 106K/&OHgr; cm. A decrease in &sgr; resulted from electrodiffusion (sweeping) in hydrogen. For the as‐received natural crystal, the value ofEwas 0.82 eV, but thisEvalue increased appreciably after hydrogen sweeping or after irradiation plus high‐temperature anneal. The principal observations could not be interpreted in terms of the conventional theory in which the Al3+‐M+pair and its dissociation products are the only defects involved. A modified theory, in which additional unassociated Al is present as compensation either for H+or for oxygen vacancies, allows a detailed interpretation of the results. Supporting data are also obtained from infrared measurements.