Several samples of poly(ethylene terephthalate-co-2,6-naphthalene dicarboxylate) were synthesized with different contents of dimethyl-2,6-naphthalene dicarboxylate (DMN). The polymer samples were ground into powders, and steady-state and time-resolved fluorescence measurements were carried out on these copolymers. The lifetimes of the excited DMN monomer and dimer were found to be 12 and 26.5 ns, respectively. By monitoring the overlap between the fluorescence excitation and emission spectra of the different species, it was concluded that energy migration occurs among the DMN monomers and among the DMN dimers, that energy can be transferred from an excited DMN monomer to a groundstate DMN dimer, but that energy transfer from an excited DMN dimer to a ground-state DMN monomer is unlikely. Increasing the DMN content induces the formation of energy traps for the DMN monomer fluorescence, which results in increasing DMN monomer quenching. When the DMN content reaches 25 mol %, the polymer matrix is saturated with DMN dimers, and no further quenching is observed.