This article describes the synthesis and the properties of polyesters and copolyesters prepared from ethylene glycol, terephthalic acid, 4,4' biphenyldicarboxylic acid (BDA), and 2,6-naphthlenedicarboxylic acid (NDA). The effect of incorporating varying levels of BDA and NDA on polyethylene terephthalate (PET) is described. Depending on the concentration, incorporation of BDA into PET leads to an improvement in glass transition temperature (T-g), strength, modulus, and barrier properties. Copolymers of PET containing up to about 50% BDA derived units are clear and have T-g's ranging from 85 to 105 degrees C, making them suitable for applications where a high T-g along with clarity is important. Copolymers with higher BDA concentration are highly crystalline, with high rates of crystallization from the melt. Copolymerization of NDA with oligoethyleneterephthalate leads to copolymers that are generally amorphous. Crystallinity can be developed in copolymers with low concentration of NDA by thermal annealing.