Poly(ethylene isophthalate-co-ethylene terephthalate) (PEIPET) copolymers of various compositions and molecular weights were synthesized by melt polycondensation and characterized in terms of chemical structure and thermal and rheological properties. At room temperature, all copolymers were amorphous and thermally stable up to about 400 degreesC. The main effect of copolymerization was a monotonic increase of glass transition temperature (T-g) as the content of ethylene terephthalate units increased. The Fox equation accurately describes the L-composition data. The presence of ethylene terephthalate units was found to influence rheological behavior in the melt, with the Newtonian viscosity increasing as the content of ethylene terephthalate units increased. (C) 2004 Wiley Periodicals, Inc.