Rates of epoxy-amine reactions in thermoset/thermoplastic blends with different thermoplastic concentrations were studied. The selected thermoset system was bisphenol A diglycidyl ether cured with 4,4＇-methylenebis[3-chloro-2,6-diethylaniline] in the presence of various concentrations of polyetherimide (PEI) (10-64 wt %). As the initial systems are homogeneous, the rate constants for the epoxy-amine neat system and the rate constants of the blends are the same, the addition of PEI in the epoxy-amine system leads only to a dilution of reactive groups during this first step. But due to the epoxy-amine reaction, a liquid-liquid phase separation occurs at a given conversion (in the range 0.2-0.4). At the same time and for PEI concentrations equal or higher than 30 wt %, a sudden increase of the reaction rate is observed from experimental results. Comparison between blends based on PEI and polystyrene (PS) shows that after phase separation the reaction rate of the PS blend was higher than the PEI blend due to a faster phase separation process for the PS blend. Estimations of the new dilution ratio of the two phases have been made through glass transition temperature measurements. Modeling of the kinetics of the epoxy-amine reaction in each phase of a 48 wt % PEI blend have been made. The increase of the reaction rate at phase separation can be explained by the formation of an epoxy-amine-rich phase with a faster reaction rate. Modeling permits to predict that gelation in this phase occurs at a conversion close to 0.6.