The fluorescence behavior of poly(ethylene terephthalate-co-ethylene-2,6-naphthalene dicarboxylate) containing 1 mol % ethylene naphthoate is investigated. The low content of naphthoic units allows to neglect excimer formations between two naphthoic moieties. Thus, the fluorescence of monomeric naphthoic units embedded in a PET matrix is analyzed. The naphthoic units act as traps for the PET fluorescence. When exciting the terephthalic units, fluorescence of naphthoic units is observed. Time-resolved fluorescence and fluorescence depolarization experiments show that excitation energy transfer takes place from terephthalic units to naphthoic traps. This energy transfer leads to a distribution of decay times. In the case of direct excitation of naphthoic units above the absorption range of terephthalic units, also nonexponential fluorescence decay curves are observed, indicating interactions of naphthoic units with the PET matrix.