The cure-induced phase separation approach for thermoplastic toughened thermosets of epoxy/DDS (4,4'-diaminodiphenyl sulfone)/PEK-C (modified polyetherketone) is studied by in situ rheology, real-time TOM (transmission optical microscope) in wide time-temperature space. Rheology studies show that there are two critical gel transformations corresponding to the loose entangled thermoplastic network at the threshold of phase separation and the percolated denser thermosetting network at chemical gelation point, respectively, within broad cure temperature range. The phase separation time may vary by using rheological or morphological methods, but its temperature dependencies can be well described by Arrhenius equation with similar phase separation activation energy E-a(ps). It is found in the present systems that E-a(ps) keeps unchanged with varying PEK-C content and different epoxy monomers, while changes along the stoichiometric ratio of hardener, which presumably affects the chemical environment of the blends. The quantitative description of time/temperature dependence of phase separation is of practical importance for the design of cure paths in processing and optimizing the properties of TP/TS composites. (c) 2008 Wiley Periodicals, Inc.