The study of the effect of intensity of illumination on the dispersion of the complex dielectric constant of a phosphor leads to the conclusion that the dielectric constant and dielectric loss of the phosphor are due to the effect of photoconducting grains separated by insulating boundaries rather than the polarizability of trapped electrons. A review of the theory of dielectric dispersion shows that these two mechanisms are equivalent to explain the experimental behavior of the phosphor but that the effect of increasing the light intensity is to decrease the grain conductivity and hence the relaxation time on one hypothesis, but can only affect relative distribution of electrons in traps on the other. The latter is rejected as inadequate to explain a ten-fold decrease in relaxation time for a four-fold increase in intensity. The complex dielectric constant and its dispersion is also studied as a function of activator concentration and temperature. These measurements are discussed from the above point of view.