The phase structure of poly(ethylene terephthalate)/poly(ethylene 2,6-naphthalate) (PET/PEN) blends was studied in relation to the molecular weight. The samples were prepared by both solution blends, which showed two glass-transition temperatures (T-g), and melt blends (MQ), which showed a single Tg, depending on the composition of the blends. The T-g of the MQ series was independent of the molecular weight of the homopolymer, although the degree of transesterification in the blends was affected by the molecular weight. The MQ series showed two exotherms during the heating process of a differential scanning calorimetry scan. The peak temperature and the heat flow of the exotherms were affected by the molecular weight of the homopolymers. The strain-induced crystallization of the MQ series suggested the independent crystallization of PET and PEN. Based on the results, a microdomain structure of each homopolymer was suggested. (c) 2005 Wiley Periodicals, Inc.