The phase separation process in a thermoplastic-modified epoxy system was studied using rheological dynamic analysis (RDA) and differential scanning calorimetry (DSC). Transmission electronic microscopy (TEM) was used to get direct representation of morphologies at different times during the phase separation process. The selected thermoset system was bisphenol A diglycidyl ether (DGEBA) cured with 4,4＇-methylenebis[3-chloro,2,6-diethylaniline] (MCDEA) in the presence of various compositions of polyetherimide (PEI), 10-64 wt %. As rheology is a signature of connectivity, the rheological behavior at phase separation was found to be greatly dependent on the initial concentration of PEI. Experimental results showed that when PEI concentration was lower than 10-15 wt %, phase separation induced a rapid decrease of the viscosity. For concentrations close to the phase inversion composition, a rheological behavior characteristic of a bicontinuous morphology appeared with a strong dependence on frequency. When PEI concentration was higher than 30 wt %, phase separation led to a gradual increase in viscosity. A large interdependence between morphology and initial composition was exhibited. Modeling of the rheological behavior using kinetic parameters and the relationship between viscosity and mass average molar mass was in good agreement with the experimental results.