This article deals with a new way of improving the melt viscosity of linear poly(ethylene terephthalate) (PET) chains through the reaction of the PET end groups (alcohol and acid) with new chain extenders, 3-(triethoxysilyl)propylsuccinic anhydride (ASSI) and 3-glycidoxypropyltrimethoxysilane, during the melt processing of PET. The reactions, investigated with model compounds monomethylterephthalate and triethylene glycol monomethylether for PET-COOH and -OH end groups, respectively, by multinuclear NMR spectroscopy (H-1, C-13, and Si-29), provided evidence of well-known acid-epoxide and alcohol-anhydride reactions, respectively. In addition, numerous other species appeared because of the presence of alkoxysilane groups, such as alcohol-alkoxysilane exchange reactions, acyloxysilane formation, and hydrolysis- condensation reactions of alkoxysilane. All these reactions led to the formation of branched chains when transposed to PET melt modification. A size exclusion chromatography analysis and the rheological behavior confirmed the presence of branched structures embedded in shorter linear PET chains. The rheological behavior of this blend was drastically modified in comparison with that of neat PET; consequently, there was an important increase in the zero-shear viscosity, with a maximum concentration of branched structures of about 17 Vol % obtained with an ASSI/PET molar ratio of 4. © 2005 Wiley Periodicals, Inc.