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The instantaneous fluorescence quantum yield φins—a new quantity for fluorescence studies defined as the ratio of the fluorescence intensity to the optical density, both measured at the moment of the maximum of the exciting pulse—proves to be a very sensitive function for excited-state processes. Dependent on the excitation intensity φins exhibits characteristic features (maxima/minima) indicating, for example, excited-state absorptions and annihilation processes. φins is therefore more informative as the intensity dependence of the usually utilized fluorescence yield, the information content of which is restricted because this function is hardly structured. In the paper the influences of specific molecular parameters (excited-state absorption cross section, annihilation constant) on φins are given, problems of the experimental accessibility of φins are discussed, and an experimental setup for determination of this new quantity is presented. The application of the method is demonstrated for identification of excited-state absorptions of organic molecules in solution.