At low contents of the manganese activator, the decay of phosphorescence takes place in two stages. The initial stage is exponential. It is complete in less than 0.1 second and is not affected by temperature. The second stage is at a much lower intensity level. It becomes noticeable after 0.03 second and persists for more than an hour. Its speed of decay increases with temperature. At - 196°C it is completely latent. A change in the intensity of the exciting light alters proportionally the luminescence caused by each stage of decay and has no effect upon the shape of the decay curves. About fifty times as many electrons are involved in the second stage of decay as in the first. It is assumed that the second stage is caused by the retention of electrons in a metastable state, probably one associated with the presence of lattice imperfections. The exponential decay of the first stage would result from the recombination of manganese ions, relatively in excess, with those free electrons which had not been captured in the potential cups of the metastable state. The decay of phosphors rich in manganese is always very abrupt and at a speed which rises rapidly with increase in temperature. The course of decay can be represented by the expression p-x=a+bt except at reduced temperatures where the two stages of decay characteristic of low manganese content are observable.