The objective of this article is to study the combined effect of isophthalic acid (IPA) and polyethylene glycol (PEG-400) in PET polymer and film on thermal, mechanical, and gas transport properties. The purpose of developing this material is to reduce the melting point, improve mechanical, thermal, and gas barrier properties. The chosen raw materials, namely, IPA and PEG for copolyester synthesis will replace partially the acid and diol monomers of PET. The molar concentration of comonomers (IPA and PEG-400) were varied from 2 to 50% and the result shows that the gas barrier properties (namely O2, CO2, N2, and water vapor transmission rate), mechanical, and thermal properties were lesser than that of PET polymer. On improving the crystallinity of PET-isophthalate-PEG (PET-IP) copolymer, barrier properties are improved than that of PET polymer. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012