An advanced barrier material was produced by using antiplasticization with poly(ethylene terephthalate) (PET). The incorporation of low molecular weight additives into PET led to antiplasticization characterized by reduction in both excess free volume and glass transition temperature. It also reduced O2 permeability. Solid state 13C-NMR experiment demonstrated a reduction in local chain mobility of carbonyl carbon. The Flory interaction parameter, estimated by Materials StudioŽ produced better estimation of interaction forces between PET and each diluent than those estimated by solubility parameters. The permeability of CO2 in PET and each modified PET was also studied. Dynamic Mechanical analysis (DMA) was employed to determine the mechanical property changes in modified PET.