Low temperature thermoluminescence was used to characterize ZnO powders with and without dopant, grown by different tehniques, before and after calcination. Native and extrinsic defects present near the surface oh the solid can be seen by this technique. Their energy is of the same order of magnitude as that seen by elec. measurements on sintered ceramics. This work is a first concerning the relation between the initial powder and the deep levels in varistors.