Electroluminescence intensity of phosphor layers decreases with running time; this decay can be described by the empirical expression, proposed by Roberts,L=L0/(1+t/t½), wheret½is the time to half‐intensity. It is also observed that, during electroluminescence decay at room temperature, (1) photoluminescence remains constant, (2) a component of the dark capacitance decreases in proportion to the light output, (3) photoconductivity and dark conductivity show a similar decrease, and (4) the light output wave form shows marked progressive changes. Also, (5) decay at 80°K proceeds at a much slower rate. An approximate derivation of an expression describing trap or donor depletion by electrolysis serves to correlate the data and the empirical expression in the foregoing.