The incorporation of hydroxy-functionalized hydrogenated polybutadienes (HO-PEB-OH, KRATON liquid polymer) into PET-based copolyesters by a conventional two-step melt polycondensation procedure is described. The usually occurring macrophase separation with nonpolar soft segments is avoided by chain extension of HO-PEB-OH with ethylene oxide to yield the corresponding hydroxy-terminated PEO-PEB-PEO triblock copolymers. Several PEO-PEB-PEO triblock copolymers with different PEO block lengths have been synthesized by means of anionic synthesis and incorporated into PET-based copolyesters with varying PET content. We show that the chain extension of HO-PEB-OH with ethylene oxide compatibilizes the nonpolar KEATON with the polar reactants 1,4-butanediol and dimethyl terephthalate during melt polycondensation, leading to a complete incorporation of the triblock copolymer into the copolyester. Morphological studies using SFM as well as mechanical testing show that the morphology is strongly influenced by the soft segment leading to dispersed PET crystallites in a matrix of the soft phase. Thermal characterization of the synthesized copolyesters by DSC exhibits a low glass transition temperature and a high PET melting point even at high soft segment contents, making these materials suitable for low- and high-temperature range applications.