The effect of prolonged annealing treatments at elevated temperatures (900–1700 K) on the a.c. ionic conductivity of Sawyer PQ ‘hydrogen swept’ quartz has been investigated by impedance spectroscopy measurements. Transient and irreversible effects have been observed during the annealings: specifically, a monotonic enhancement of the conductivity is detected in the 900–1300 K temperature range, while at higher annealing temperatures an opposite effect is evidenced. The experimental results can be interpreted in the frame of a phenomenological model featuring two equilibrium reactions; an indirect role of hydrogen impurity in the ionic transport dynamics is evidenced, also by means of infrared spectroscopy measurements.