The decay of phosphorescence of various types is measured during the first few milliseconds of the process. Phosphorescence with exponential decay occurs when luminescence is due to an optical transition of a type normally forbidden. Experimental proof of this is provided by a study of the ruby. A further study is made of the effect of temperature on exponential decays. Some phosphors give complex decays in which a temperature-dependent decay process due to thermally metastable states or traps is superimposed on the exponential decay. An experimental separation of these decay processes is described for the case of ZnS-Mn. It had been believed that in hyperbolic decays, characteristic of ZnS-Cu and ZnS-Ag, the time constant of the process was due to the time that an excited electron spent in moving through the phosphor. The experiments described here show, however, that this type of decay is due primarily to the time electrons spend in traps; therefore the bimolecular theory of phosphorescence is largely rejected. The main experimental results show the temperature dependence of the decay curve and the correlation between the decay curve and the amount of light energy which may be stored in traps in the phosphor at low temperatures. An explanation is given of the change of shape of the decay curve with temperature.