The form of luminescence decay curves are discussed based on the nature of inter molecular interaction between the pertinent excited state molecule and its surrounding molecules. This study shows that the effets of excitation duration and light intensity differ with the types of interaction. Five theoretically derived patterns are shown. The validity of these patterns are verified by comparing them with those obtained experimentally. As the pattern of the decay curve corresponds to the type of interaction, in some cases the operative interaction can be revealed experimentally by observing the behavior of luminescence decay curves under various excitation conditions. In order to illustrate the usefulness of the method, an example is given of the successfuI application of the present method to an outstanding problem.