Poly(ethylene tereplithalate-co-isophthalate-co-5-tert-butylisophthalate) ((PETIBI)-B-t) terpolymers were investigated with reference to poly(ethylene terephthalate) (PET) homopolymer and poly(ethylene terephthalate-co-isophthalate) (PETI) copolymers. Three series of (PETIBI)-B-t terpolyesters, characterized by terephthalate contents of 90, 80, and 60 mol %, respectively, with different isophthalate/5-tert-butylisophthalate molar ratios, were prepared from ethylene glycol and mixtures of dimethyl terephthalate, dimethyl isophthalate, and 5-tert-butylisophthalic acid. The composition of the terpolymers and the composition of the feed agreed. All terpolymers had a random microstructure and number-average molecular weights ranging from 10,000 to 20,000. The (PETIBI)-B-t terpolyesters displayed a higher glass-transition temperature and a lower melting temperature than the PETI copolymers having the same content of terephthalic units. Thermal stability appeared essentially unchanged upon the incorporation of the 5-tert-butylisophthalic units. The PETI(t)BIs were crystalline for terephthalate contents higher than 80 mol %, and they crystallized at lower rates than PETI. The crystal structure of the crystalline terpolymers was the same as that of PET with the 1,3-phenylene units being excluded from the crystalline phase. Incorporation of isophthalate comonomers barely affected the tensile modulus and strength of PET, but the brittleness of the terpolymers decreased for higher contents in 5-tert-butylisophthalic units.