In this article we present the results of a study on the properties of a blend containing a 40:60 amine-ended copolymer, poly(ether sulfone)-poly(ether ether sulfone), with a system composed of diglycidyl ether of biphenol S and 4,4'-diaminodiphenyl sulfone (4,4'-DDS). Five formulations, which varied in amounts of modifier, were characterized through dynamic thermal mechanical analysis and fracture mechanics studies. The morphology of the blends was studied by transmission electron microscopy (TEM). The addition of thermoplastics enhanced the toughness of the system, resulting in an increase in the glass-transition temperature as a function of the amount of thermoplastics. No phase-separated morphology was detected by TEM analysis, leading to the conclusion that the formation of a homogeneous semi-IPN network occurred between the thermoplastics and the thermoset. (C) 2003 Wiley Periodicals, Inc.