AbstractThermoplastic‐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‐separated macrostructure and also facilitated more complete phase separation. The intermediate dwell helped phase separation 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 fracture toughness than the slowly heated material. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009