Low molecular weight thermoplastic PES : PEES (polyethersulfone : polyetherethersulfone) copolymer, bearing amine reactive end groups, was blended with a diglycidyl ether of bisphenol A (EPON828) epoxy resins. Two different curing agents, MDEA [methylene bis(2,6‐diethylaniline)] and 3,3′DDS (3,3′‐diamino diphenyl sulfone), were used separately to compare flow behavior, curing kinetics, and morphology of the final samples. Emphasis was placed on different phase behaviors of the cured systems, attributed to different solubilities of thermoplastic in the thermoset system as well as to the reactivity of the two curing agents. The presence of PES : PEES was shown not to affect the reaction rate of epoxy blends with respect to the neat resin by the reactive nature of its end groups. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 361–371, 2004