A series of random copolyesters was prepared by replacing up to 10 wt.% of the dimethyl terephthalate (DMT) in poly(ethylene terephthalate) (PET) with dimethyl 2,6-naphthalene dicarboxylate (NDC), isophthalic acid (IPA), or 2,5-bis-(4-carboxyphenyl)-1,3,4-oxadiazole (ODCA). Solution cast films of the resulting copolymers were prepared and characterized. Modification of PET with NDC and ODCA led to copolymers with glass transition temperatures higher than that of PET, while modification with IPA decreased the glass transition temperature. Copolymerization decreased crystallinity levels in all cases. The acetone solubility and acetone diffusion coefficient were determined by integral kinetic gravimetric sorption experiments at 35 degrees C and 5.4 cm Hg acetone pressure. PET containing low levels of NDC had lower amorphous phase acetone diffusivity and solubility than PET, while PET modified with IPA had amorphous phase acetone diffusivity and acetone solubility similar to that of PET. PET modified with 5% ODCA had amorphous phase acetone diffusivity similar to that of PET, while PET modified with 10% ODCA had an amorphous phase acetone diffusivity value slightly lower than that of PET. Copolymers containing ODCA had somewhat higher acetone solubilities that PET, due mainly to the lower levels of crystallinity in the ODCA-containing polymers than in PET.