Binary copolymers of poly(ethylene 2,6-naphthalate) (PEN) and poly(hexamethylene 2,6-naphthalate) (PHN) were synthesized and their sequence distributions were investigated over the entire range of copolymer composition by C-13 nuclear magnetic resonance spectroscopy. The melting temperatures of these copolymers were depressed gradually with the increase of 1,6-hexanediol (HD) in the composition and eutectic behaviour appeared which depended only on the sequence propagation probability P, not directly on the composition, and showed a minimum at about 60 mol% HD content at which the average sequence length is the shortest. The copolymers rich in ethylene units formed only PEN crystals with complete rejection of the hexamethylene units. Also, the copolymers rich in hexamethylene units formed only PHN crystals excluding ethylene units.