The influence of incorporating 5-tert-butyl isophthalic units ((BI)-B-t) in the polymer chain of poly(ethylene terephthalate) (PET) on the crystallization behavior, crystal structure, and tensile and gas transport properties of this polyester was evaluated. Random poly(ethylene terephthalate-co-5-tert-butyl isophthalate) copolyesters ((PETBI)-B-t) containing between 5 and 40 mol% of (BI)-B-t units were examined. Isothermal crystallization studies were performed on amorphous glassy films at 120degreesC and on molten samples at 200 degreesC by means of differential scanning calorimetry. Furthermore, the non-isothermal crystallization behavior of the copolyesters was investigated. It was observed that both crystallinity and crystallization rate of the (PETBI)-B-t copolyesters tend to decrease largely with the comonomeric content, except for the copolymer containing 5 mol% of (BI)-B-t units, which crystallized faster than PET. Fiber X-ray diffraction patterns of the semicrystalline (PETBI)-B-t copolyesters proved that they adopt the same triclinic crystal structure as PET with the comonomeric units being excluded from the crystalline phase. Although (PETBI)-B-t copolyesters became brittle for higher contents in (BI)-B-t, the tensile modulus and strength of PET were barely affected by copolymerization. The incorporation of (BI)-B-t units slightly increased the permeability of PET, but copolymers containing up to 20 mol% of the comonomeric units were still able to present barrier properties.