The effect of antiplasticization on barrier properties of poly(ethylene terephthalate) (PET) has been investigated through transport measurements and some supplementary characterization techniques including dynamic mechanical measurements and solid state 13C cross polarization/magic angle spinning (CP/MAS) NMR. Our oxygen and carbon dioxide transport results were well described by a combination of the free volume based interpretation and interaction energy estimation. Transport characterization and DSC techniques verified that there exists a third element, a de-densified amorphous fraction in crystallized PET. Both oxygen and carbon dioxide permeabilities at 1 atm at 35ºC in PET with different crystallinities were well described by the Nielsen model due to the presence of an adjustable parameter, Ar, even though it is based on a two phase model. The comparison of the barrier improvement factor (BIF) values for samples annealed at 100ºC demonstrated that a combination of antiplasticization and crystallization allows for very efficient chain packing, which significantly improves the barrier properties of PET. A thorough molecular level study using dynamic mechanical analysis supported the synergistic effect of antiplasticization and crystallization on the molecular motion in PET. Lastly, a vapor/gas permeation system with a new concept of a flexible humidity and methanol vapor partial pressure clamp was designed and constructed. Even though its permeation results are not available at this point, its operational feasibility was well verified by pre-calculations and physical explanations. This system may be used for future studies to evaluate barrier properties of PET or modified PET samples.