Segmented copolyesters, namely, poly(butylene terephthalate)-poly(ethylene terephthalate-co-isophthalate-co-sebacate) (PBT-PETIS), were synthesized with the melting transesterification processing in vacuo condition involving bulk polyester produced on a large scale (PBT) and ternary amorphous random copolyester (PETIS). Investigations on the morphology of segmented copolyesters were undertaken. The two-phase morphology model was confirmed by transmission electron microscopy and dynamic mechanical thermal analysis. One of the phases was composed of crystallizable PBT, and the other was a homogeneous mixture of PETIS and noncrystallizable PBT. (C) 2003 Wiley Periodicals, Inc.