The effects of reactive melt modification of poly(ethylene terephthalate) (PET) with a multifunctional epoxide-triglycidyl isocyanurate-that lead to chain extension/branching and formation of gel-like structures were rheologically characterized. The storage and loss moduli and the complex viscosity of the modified PET were larger than those of the unmodified PET. The elastic or solidlike behavior of PET was enhanced after reactive modification as a result of chain extension/branching. Modified Cole-Cole plots revealed that the modified resins show higher elasticity than the unmodified one. Reactive modification characterized by the presence of long-chain branching resulted in a wider molecular weight distribution. The effects of mixing temperature and the concentration of modifier corresponding to different stoichiometries were investigated. Higher amounts of modifier resulted in a polymeric structure near the sol-gel transition point whose linear viscoelastic properties obey scaling law. The relaxation spectrum was calculated from the oscillatory shear data by Tschoegl equations. Certain modified resins have rheological properties suitable for process operations such as extrusion foaming, blow molding, and thermoforming. (C) 2003 Wiley Periodicals, Inc.