Copoly(ethylene terephthalate-imide)s (PETIs) were synthesized by the melt copolycondensation of bis(2-hydroxyethyl)terephthalate with a new imide monomer, N,N'-bis[p-(2-hydroxyethoxycarbonyl)phenyl]-biphenyl-3,3',4,4'-tetracarboxydi- imide (BHEI). The copolymers were characterized by intrinsic viscosity, Fourier transform infrared, H-1 NMR, differential scanning calorimetry, and thermogravimetric analysis techniques. Although their crystallinities decreased as the content of BHEI units increased, the glass-transition temperatures (T-g) increased significantly. When 5 or 10 mol % BHEI units were incorporated into poly(ethylene terephthalate), T-g increased by 10 or 24 degreesC, respectively. The thermal stabilities of PETI copolymers were about the same as the thermal stability of PET, whereas the weight loss of PETIs decreased as the content of BHEI units increased. (C) 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 408-415, 2001.