Poly(butylene terephthalate-co-triethylene terephthalate) random copolymers of various compositions and molecular weights were synthesized in bulk and characterized in terms of their chemical structure and thermal and rheological properties. At; room temperature all the copolymers were partially crystalline and showed good thermal stability. The main effect of copolymerization was a decrease in the melting and glass-transition temperatures with respect to the poly(butylene terephthalate) homopolymer. The fusion temperatures were well correlated with the composition by the Baur equation and the equilibrium melting temperature and the heat of fusion extrapolated values for poly(butylene terephthalate) were in good agreement with those reported elsewhere. Triethylene terephthalate units were found to influence the theological behavior in the melt, the viscosity being significantly higher than that of the poly(butylene terephthalate-co-diethylene terephthalate) copolymers investigated previously.