The paper describes measurements of the electric strength of crystals of KCl and NaCl which were made using both d c. and standard 1/50 μsec unidirectional impulse voltages at temperatures between - 74° c and + 140° c. In this range of temperature, which included for both KCl and NaCl a transition from a region in which the electric strength increased with rise in temperature to one in which it decreased, no significant difference was found between values of electric strength obtained using d.c. and corresponding values obtained with impulses. This result contrasts with that of Suita for KCl. It supports qualitatively Frohlich's theory of breakdown.