Toughened mixtures containing 15 wt % polyethersulfone were made with diglycicdyl ether of bisphenol-A resin and 4,4＇-diaminodiphenylmethane curing agent, with amine/epoxy group stoichiometric ratios varying from 0.6 to 1.5. Fracture behavior of the modified mixtures has been investigated as a function of the stoichiometry in the matrix. Morphology has been analyzed by transmission and scanning electron microscopy. The increase of amine content in the matrix results in a further increased fracture toughness. This behavior has been related to the changes on the ductility of the matrix upon stoichiometric ratio, but also to the changes on microstructural features of the modified mixtures as stoichiometric amine/epoxy group ratio increased. These morphological changes have been interpreted in terms of spinodal decomposition during curing of the epoxy matrix.