Copoly(ethylene terephthalate-imide)s (PETI) were prepared by melt polycondensation of bis(2-hydroxyethyl)terephthalate (BHET) and imide containing oligomer, i.e., 4,4’-bis[(4-carbo-2-hydroxyethoxy)phthalimido]diphenylmethane (BHEI). The apparent rate of polycondensation reaction was faster than that of homo poly(ethylene terephthalate) (PET) due to the presence of imide units. The PETI copolymers with up to 10 mol % of BHEI unit in the copolymer showed about the same molecular weight and carboxyl end group content as homo PET prepared under similar reaction conditions. The increase in T-g of copolymer was more dependent on molar substitution of BHEI than on substitution of BHEN, reaching 91 degrees C with 8 mol % BHEI units in the copolymer from T-g = 78.9 degrees C of homo PET. In the case of PETN copolymer, 32 mol % of bis(2-hydroxyethyl)naphthalate (BHEN) units gave T-g of 90 degrees C. The maximum decomposition temperature of PETI copolymer was about the same as that of homo PET by TGA analysis. The char yield at 800 degrees C was higher than that of homo PET.