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The fluorescence spectra of pyrene and some of its derivatives vary strongly with the concentration of these compounds in their solutions. With pyrene itself in dilute solution the spectrum is violet and has a well defined structure. With increasing concentration a second fluorescence component without structure appears in the blue region, whereas the violet component decreases more and more. The half-value concentration of this change varies with the solvent viscosity, being $3\times 10-4$ molar in n-pentane and $1.2\times 10-2$ molar in liquid paraffine. In all solutions investigated, Beer’s Law is valid within the whole concentration interval. The blue component is not identical with the fluorescence of the crystalline compound. No photoreaction is observed. The fluorescence change is attributed to a diffusion-controlled process between an excited pyrene molecule and an unexcited one, resulting in the formation of an excited dimer molecule, which dissociates again after deactivation. The concentration dependence of both fluorescence components as well as the dependence on solvent viscosity are in favor of this interpretation. Further arguments have been derived from a study of the action of quenchers.