The dc resistivity of fused quartz with respect to Na' ions was measured at temperatures up to 750"C, using @-alumina as the sodium source. Heat treatment in air at 1100°C gave minima in resistivity at times which depended on the choice of either a high-OH or a low-OH glass. The initial fall in resistivity is explained by the stabilization of the glass to a more open structure and the later increase is attributed to a blocking effect of "permanent" hydroxyl groups, which slowly enter the glass as atmospheric water.