A reactive extrusion process was developed to toughen an amorphous copolyester (PETG) of ethylene glycol, terephthalic acid and 1,4-cyclohexanedimethanol using either a maleic anhydride grafted polyethylene-octene elastomer (POEg), or a maleic anhydride grafted mixture (TPEg) of the polyethylene-octene elastomer and a semicrystalline polyolefin plastic as the impact modifier. TPEg showed an important toughening effect on the PETG. A sharp ductile-brittle transition was observed when the TPEg content was about 10 wt %. For POEg toughened PETG, the ductile-brittle transition required a higher content in POEg, similar to 15 wt %. Evolution of the topography and morphology of the blends and the relationship between impact strength and topography were discussed.