Polyesters, prepared by direct polycondensation from bisphenol A and aliphatic dicarboxylic acids [adipic acid (AD), suberic acid, sebacic acid (SE), and dodecanedioic acid], were used to improve the toughness of the diglycidyl ether of the bisphenol A/diaminodiphenyl methane epoxy system. Polyesters had the number aver age molecular weight (M-n) ranging from 4300 to 19,200 g/mole. The epoxy systems modified with the AD system (M-n = 6400 g/mole) and the SE system (M-n = 10,200 g/mole) showed phase separated structures with discrete domains of 0.2 mum, but other systems showed smooth fracture surfaces when observed by scanning electron microscopy. The modified epoxy systems except for the AD system and SE system showed two tan delta peaks corresponding to the alpha and beta transitions of the epoxy resin. The modified epoxy systems showed maximum values of K-1c at around 10 wt % of polyester and maximum flexural properties at 5 wt % of polyester.