Dimethyl 2.6-naphthalene dicarboxylate (DMN) was embedded in a poly(methyl methacrylate) (PMMA) matrix. According to the molecular weight of the repeating unit of PMMA different samples, containing 0.02 up to 8 mol % DMN have been prepared. Starting from 1 mol % DMN content, excimer formation is observed. From time-resolved fluorescence experiments the fluorescence lifetime of the monomer was derived to be 15 ns and the excimer lifetime amounts to 30 ns. It was shown that Forster excitation energy transfer takes place among the DMN monomers and the Forster radius of the monomer was determined by time-resolved fluorescence depolarization experiments to be 23 3 A. Excitation energy transfer is assumed to be the dominant process for the excimer formation. Using a model of three-dimensional transport among DMN donors until an excimer trap is reached, dimensionless trap concentrations of 0.005 and 0.014 were derived for the samples with 1 and 2 mol % DMN, respectively. For larger DMN concentration the excitation energy transfer between DMN molecules becomes anisotropic.