This paper is concerned with the measurement and theory of long-period phosphorescence in solids. The delayed emission is, in such cases, due to the time electrons spend in traps or metastable states before returning to the luminescence centres. The theory shows how the observed decay laws are related to the trap distributions found in Paper I. In alkaline earth phosphors there are equal numbers of traps at all depths over the range considered, and the expression developed shows that the intensity of phosphorescence should be inversely proportional to time. For zinc sulphide phosphors an exponential trap distribution has been found, and this leads to a simple inverse power law for the theoretical decay curve. Measurements of decay on these phosphors utilizing an electron multiplier have confirmed the theory, and put on a quantitative basis the connexion between thermoluminescence and long-period decay. At the same time the work provides the first satisfactory and detailed explanation of long-period decays. The extent to which retrapping of electrons modifies the picture given is discussed.