Reaction-induced phase-separation behavior of an epoxy (E-51) system modified by thermoplastics, namely, polyetherimide (PEI) and polycarbonate (PC), isothermally cured with 4,4'-diaminodiphenylsulfone (DDS) at various temperatures, was investigated by the time-resolved small-angle laser light-scattering (SALLS) technique. It was found that the morphology evolution of this E-51 curing system was controlled by the competition between the curing reaction and the phase separation. It was verified that the phase separation of these systems followed the spinodal decomposition mechanism through analyzing the scattering pattern evolution and the cocontinuous morphology observed by optical microscopy. The curing temperatures and compositions of these epoxy systems were considered to be the determining factors for the domain size and the phase-separation behavior.