Frequency-dependent dielectric measurements have been used to monitor and characterize the phase separation process in high-performance thermoplastic-thermoset blends of 2,6-dimethyl-1,4-phenylene ether (PPE) with an epoxy diglycidylether of bisphenol A (DGEBA) and 4,4-methylene bis(3-chloro-2,6-diethylamine) (MCDEA). The systems studied are 30, 45 and 60% PPE/DGEBA-MCDEA blends. The results are compared with dynamical mechanical measurements and the developing morphology. Both dielectric and mechanical measurements are shown to be good techniques to monitor the phase-separation process and the reaction advancement. Dielectric measurements monitor the buildup in T-g in both the PPE-rich continuous phase and in the epoxy-rich occluded phases. Dielectric measurements are advantageous as they can be made in situ continuously on a single sample throughout the entire cure process. The results show that the phase separation process initially occurs rapidly involving a large amount of the epoxy-amine diffusing into the occluded phase. The rate of the epoxy-amine reaction in the epoxy-rich 30% PPE mixture is approximately equal to that in the neat epoxy-amine system due to two opposing effects, a slower reaction rate due to dilution and a lower level of conversion at vitrification due to the presence of high T-g PPE. In the 60% PPE mixture, the dilution effect of the PPE has a large affect on the decreasing the reaction rate and achievement of vitrification. The continuous thermoplastic-rich phase is observed to vitrify first, followed by vitrification of the thermoset as occluded particles. Finally, the results show as evidenced by the size of the occluded particles and the composition of the continuous phase that the morphology is strongly influenced by the kinetics, diffusion, and viscosity conditions during phase separation.