The influence of additives on the crystal modification and melting behavior of poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) was investigated with wide-angle X-ray diffraction and differential scanning calorimetry (DSC). The addition of a nucleating promoter, Ceraflour 993, had no effect on the crystal modification and melting behavior of PEN crystallized under all chosen experimental conditions. However, the addition of a nucleating agent, sodium benzoate (SB), did affect the crystal modification and melting behavior of PEN when PEN/SB was crystallized at a higher temperature, but not at a lower temperature. A mixture of alpha and beta modifications of PEN was obtained, and an overlapped dual melting peak was observed in DSC curves when PEN was crystallized at a higher temperature in the presence of SB, instead of a single crystal form and a single melting peak for the crystallization of pure PEN. (C) 2003 Wiley Periodicals, Inc.