Aromatic polyesters, prepared by the reaction of aromatic dicarboxylic acids and 1,2-ethanediol, were used to improve the toughness of bisphenol-A diglycidyl ether epoxy resin cured with p,p′-diaminodiphenyl sulphone. These polyesters contained poly(ethylene phthalate)s, poly(ethylene phthalate-co-ethylene isophthalate)s (PEPI), poly(ethylene phthalate-co-ethylene terephthalate)s, and poly (ethylene phthalate-co-ethylene 2,6-naphthalene dicarboxylate)s. All the aromatic polyesters used in this study were soluble in the epoxy resin without solvents and were found to be effective as modifiers for toughening the epoxy resin. For example, the inclusion of 20 wt% of PEPI (10 mol.% isophthalate unit, MW 7400) led to a 100% increase in the fracture toughness ( K IC ) of the cured resin with no loss of mechanical and thermal properties. The modified resins had a two-phase morphology and the polyesterrich dispersed particles in the epoxy matrix. The toughening mechanism was discussed in terms of the morphological and dynamic viscoelastic behaviours of the modified epoxy resin system.