Incorporation of selected low molecular weight diluents (LMWDs) into poly(ethylene terephthalate) (PET) significantly improves oxygen and carbon dioxide barrier properties. A so-called Barrier Improvement Factor (BIF), defined as the ratio of permeability of pure polymer to that of antiplasticized polymer, provides a useful indication of improved barrier properties. The average BIF was found to be 1.20 (+/- 0.02) and 1.34 (+/- 0.03) for O-2 and 1.25 (+/- 0.01) and 1.41 (+/- 0.01) for CO2 in PET containing - 2.32% phenacetin, or 1.95% acetanilide, respectively. The reduced permeability coefficient in each PET sample is attributed to antiplasticization, and justified by the combination of the effect of reduced free volume and the interaction energies of PET with LMWDs. A more compatible interaction of PET with acetanilide may require higher activation energy for diffusional jump resulting in more reduction in permeability even though both PET-2.32% phenacetin and PET-1.95% acetanilide showed an equal level of reduction in free volume. Solid-state C-13 cross polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectra verified a reduction in the localized chain mobility of carbonyl carbon of PET in both PET-2.32% phenacetin and PET-1.95% acetanilide. Furthermore, the dynamic mechanical property measurement in the low temperature region verifies that the presence of LMWDs reduced the low temperature beta relaxation in PET. (C) 2011 Elsevier Ltd. All rights reserved.