Poly(ethylene naphthalene-2,6-dicarboxylate) and poly(1,4-butylene naphthalene-2,6-dicarboxylate) homopolymers were synthesized by the two-step melt polycondensation process of dimethyl naphthalene-2,6-dicarboxylate and ethylene glycol (EG) or 1,4-butanediol (BD) respectively. Eight copolymers of the above three monomers were also synthesized by varying the mol% of BD with respect to EG in the initial monomer feed. The copolymer composition was determined by H-1 n.m.r. spectroscopy. The thermal behaviour was investigated over the entire range of copolymer composition by differential scanning calorimetry and thermomechanical analysis. The glass transition (T-g), cold crystallization (T-cc), melting point (T-m) and crystallization (T-c) temperatures have been determined. The melting temperature of the above copolymers was depressed gradually at first with the increase of ED in the composition and eutectic behaviour appeared with a minimum at about 40 mol% ED content.