Frequency dependent dielectric measurements have been used to monitor the complex chemical, composition and morphological changes occurring during cure of a high-temperature epoxy-amine (DGEBA-MCDEA) polyetherimide (PEI) blend. The dielectric measurements are able to monitor in situ the reaction advancement before phase separation, the onset of phase separation, the Maxwell-Wagner-Sillars (MWS) effect, and the buildup in T-g in both the continuous thermoplastic PEI-rich phase and the occluded epoxy-rich phase. The MWS predicted values were in relatively good agreement with the experimental values and suggest a nonspherical shape. The conductivity in the homogeneous blend decreased much more rapidly than expected based on concentration and change in viscosity, suggesting that the conduction mechanism in the homogeneous blend is strongly affected by the proximity of the hydrogen-bonding groups in the epoxy amine.