The use of multifunctional epoxy-based modifiers to increase the melt strength of poly(ethylene terephthalate) (PET) has been investigated, with the aim of producing PET foams by an extrusion process. The reaction conversion has been studied as a function of time by the measurement of torque in an internal mixer. A tetraglycidyl diamino diphenyl methane (TGDDM) resin was selected for the investigation of the modifier concentration effect on the reaction conversion. Using a stoichiometric concentration of TGDDM, the molecular weight distribution of modified PET, as determined by gel permeation chromatography (GPC), showed an eight-fold increase of the z-average molecular weight (M-z) and the presence of branched molecules of very large mass. The resulting intrinsic viscosity of the modified PET was 1.13 dl/g.