An experimental comparison has been made of three methods of measuring the unpolarized resistivity of glass and fused quartz over a temperature range from 20 to 500°C. At high temperatures all three methods are in quantitative agreement and give a straight line when the logarithm of the resistivity is plotted against the reciprocal of the absolute temperature. At lower temperatures, however, the a.c. curve deviates from this straight line, indicating an additional component of loss which does not exist on d.c. This additional loss may be due to the orientation of polar molecules, though no definite proof can be given. The results obtained by the other two methods, both with direct current, are in close agreement over the whole temperature range, and give without doubt the true unpolarized resistivity.