Poly(ethylene terephthalate-co-5-tert-butyl isophthalate) copolymers, abbreviated as (PETBI)-B-t, with compositions ranging between 95/5 and 25/75, as well as tile two parent homopolymers, PET and (PEBI)-B-t, were prepared from comonomer mixtures by a two-step melt-polycondensation. Polymer intrinsic viscosities varied from 0.4 to 0.7 dL g(-1) with weight-average molecular weights ranging between 31,000 and 80,000. The copolymers were found to have a random microstructure with a composition according to that used in the corresponding feed. The melting temperature and crystallinity of (PETBI)-B-t decreased with the content in 5-tert-butyl isophthalic units, whereas the glass-transition temperature increased from 82 degreesC for PET up to 99 degreesC fur (PEBI)-B-t. Copolymerization slightly improved the thermal stability of PET. Preliminary X-ray diffraction studies revealed that (PETBI)-B-t adopt the same crystal structure as PET with the alkylated isophthalic units probably excluded from the crystal lattice. The homopolymer (PEBI)-B-t appeared to be a highly crystalline polymer taking up a crystal structure clearly different from that of PET and (PETBI)-B-t copolymers.