Rubbers of different kind were tested as toughening agents of poly(ethylene terephthalate) (PET), noting significant morphological and mechanical differences. In particular, good results were obtained by using an ethylene-ethyl acrylate-glycidyl methacrylate copolymer. The resulting blend evidenced good particle distribution, and the latter was related to chemical interactions between the rubber epoxy groups and PET terminal groups, including the effect of low molecular weight and polymeric amine catalysts, and to extrusion conditions.