Active barrier developments for poly(ethylene terephthalate) (PET) are among important approaches being used to achieve improvements in the polyester's barrier properties. This work utilized two active barrier materials and their PET copolymers, synthesized by melt polymerization of terephthalic acid (TPA) with ethylene glycol (EG) and active oxygen scavengers: monoolein (MO) or 3-cyclohexene-1,1-dimethanol (CHEDM), in separate compositions. The oxidizable monomers and their copolymers were characterized with respect to changes resulting from their reactions with oxygen. Evaluations of their infrared and nuclear magnetic resonance spectra established that the monomers as well as their PET copolymers underwent first order reactions and had rate constants ranging from 0.03 to 0.08 day(-1). The only volatile oxidation product of MO was acetaldehyde and no volatile products resulted from oxidation of CHEDM. In comparison to unmodified PET, the PET/MO and PET/CHEDM copolymers achieved respective oxygen barrier property improvements of about 30 and 40%. Results from this study have shown that the oxidation kinetics of pure monomers of this type may be used for evaluation of their effectiveness and suitability as active barrier materials, without the need to synthesize their corresponding PET copolymers. This is significant and work is continuing to include other moieties that will interact with higher levels of oxygen and react at faster rates. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4273-4283, 2013