The present invention relates to thermoplastic polyesters, which comprise, by weight: (i) a thermoplastic polyester; (ii) an impact modifier comprising: (a) a core-shell copolymer (A); (b) an ethylene copolymer (B) chosen from ethylene-unsaturated carboxylic acid anhydride copolymers (B1), ethylene-unsaturated epoxide copolymers (B2) and blends thereof; (iii) the (B)/(A) ratio being between 40/60 and 10/90 for proportions of impact modifier between 18 and 40% in 82 to 60% of polyester, respectively; (iv) the (B)/(A) ratio being between 40/60 and 25/75 for proportions of impact modifier between 2 and 18% in 98 to 82% of polyester, respectively. It is particularly useful for PET and PBT.