An epoxy resin system based on a triglyeidyl p-amino phenol (MY0510) was crosslirtked using stoichiometric amounts of 4,4'-diaminodiphenyl sulfone. The epoxy was modified with random copolymers, polyethersulfone-poly(ether- ethersulfone) (PES:PEES), with either amine or chlorine end groups, at 10 and 20 wt . The reaction kinetics for both unmodified and modified epoxy systems were studied using differential scanning calorimetry in isothermal and dynamic conditions. The results show that the degree of conversion in thermoplastic-modified epoxies at any reaction time is smaller compared with the unmodified resin. Gel point (GP) determination was done from rheological measurements. The modified system containing 20 of the PES:PEES additive showed considerable increase in the GP. The reaction rate shows the characteristic of an autocatalytic reaction where the product acts as catalyst. The activation energy, Ea calculated from the isothermal reaction depends on the extent of conversion and increases with increasing PES:PEES content. For unmodified epoxy system, the average Ea is 67.8 ± 4.1 kj mol-1 but for systems modified with 20 wt of amine and chlorine PES:PEES, the value increased to 74.1 ± 3.3 and 77.9 ± 4.4 kj mol-1, respectively. © 2009 Wiley Periodicals, Inc.