Thermoplastic-toughened epoxy resins are widely used as matrices in modern composite prepreg systems. Rapid curing of thermoplastic-toughened epoxy matrix composites results in different mechanical properties. To investigate the structure-property relationship, we investigated a poly(ether sulfone)-modified triglycidylaminophenol/4,4'-diamino diphenyl sulfone system that was cured at different heating rates. An intermediate dwell was also applied during the rapid heating of the thermoplastic-modified epoxy system. We found that a higher heating rate led to a larger domain size of the phase-seperated macrostructure and also facilitated more complete phase seperation. The intermediate dwell helped phase seperation to proceed even further, leading to an even larger domain size of the macrostructure. A carbon-fiber-reinforced polymer matrix composite prepreg based on the poly(ether sulfone)-modified multifunctional epoxy system was cured with the same schedule. The rapidly heated composite laminates exhibited higher mode I delamination. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 485-491, 2009