Epoxy based on diglycidyl ether of bisphenol A + 4,4 ' diaminodiphenylsulfone blended with poly(vinyl acetate) (PVAc) was investigated through differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and environmental scanning electron microscopy (ESEM). The influence of PVAc content on reaction induced phase separation, cure kinetics, morphology and dynamic-mechanical properties of cured blends at 180 C is reported. Epoxy/PVAc blends (5, 10 and 15 wt % of PVAc content) are initially miscible but phase separate upon curing. DMTA a-relaxations of cured blends agree with T-g results by DSC. The conversion-time data revealed the cure reaction was slower in the blends than in the neat system, although the autocatalytic cure mechanism was not affected by the addition of PVAc. ESEM showed the cured epoxy/PVAc blends had different morphologies as a function of PVAc content: an inversion in morphology took place for blends containing 15 wt % PVAc. The changes in the blend morphology with PVAc content had a clear effect on the DMTA behavior. Inverted morphology blends had low storage modulus values and a high capability to dissipate energy at temperatures higher than the PVAc glass-transition temperature in contrast to the behavior of neat epoxy and blends with a low PVAc content. (c) 2006 Wiley Periodicals, Inc.