Some properties of phosphors under 3650 Å excitation are described, viz., the thermal glow, decay, and buildup of fluorescence, temperature dependence, and light sum. Most of the experimental results can be explained by a model in which cobalt levels act both as electron traps with a trap depth of 0.5 electron volt (ev), and as acceptors for holes, ejected thermally from copper centers with an activation energy of 1.1 ev. The possibility of excitation by 3650 Å radiation of electrons from traps to the conduction band is introduced to explain the observed intensity dependence of the light sum.