Abstract: The impact strengths of linear polyesters such as poly(ethylene terephthalate) and poly(1,4-butylene terephthalate) are improved, particularly at low temperatures, by the incorporation therein of a core-shell impact modifier. The core therein is a crosslinked elastomeric polymer of at least one conjugated diene such as butadiene and has a volume average particle size in the range of about 90-700 nanometers. The grafted shell is a copolymer of at least one vinyl aromatic compound such as styrene and at least one polymerizable nitrile such as acrylonitrile.