A comparative study of the two isophthalic acid deriving homopolyesters poly(ethylene isophthalate) (PEI) and poly(ethylene 5-tert-butyl isophthalate) ((PEBI)-B-t), including synthesis, crystal structure, and thermal and permeability properties, was carried out. The two polyesters were prepared by condensation polymerization in the melt. In both cases, minor amounts of cyclic dimers were observed to form, which were characterized by nuclear magnetic resonance and mass spectroscopy. PEI and (PEBI)-B-t were thermally stable up to 400degreesC and they appeared to be semicrystalline polyesters, having their melting temperatures between 130 and 135degreesC. Their glass-transition temperatures were 62 and 94degreesC, respectively. The crystal structure adopted by the two polyesters seemed to consist of a regularly folded conformation, clearly different from the almost extended conformation characteristic of poly(ethylene terephthalate). Gas permeability measurements for N-2, O-2, and CO2 revealed that (PEBI)-B-t is more permeable to these gases than PEI, in spite of having a higher T-g Furthermore, water vapor diffusion was found to be increased by the insertion of the tert-butyl group, whereas water absorption diminished. The differences in gas and water vapor transport properties observed for these two polyesters were discussed on the basis of their respective molecular structures. (C) 2004 Elsevier Ltd. All rights reserved.