Energy-filtering transmission electron microscopy (EFTEM) was employed to investigate the morphology developments of thermoset/thermoplastic blends of poly(2,6-dimethyl-1,4-phenylene ether)/bis(vinylphenyl) ethane, PPE/BVPE. Neither conventional TEM images nor energy-filtered images at any energy loss levels showed any evidences for the phase separation of the blends, while those could be shown and characterized by the oxygen maps representing the differences in the oxygen concentrations between the two phases, which could be characterized as the PPE-rich phase (oxygen-rich phase) and the BVPE-rich phase (oxygen-poor phase). The blends undergo phase decomposition into the two phases through the crosslinking reaction of BVPE. The effect of the composition and the curing time on the phase decomposition behaviors was investigated by quantitative analysis of EELS spectra acquired from the two phases. Reactive functional moiety was introduced onto PPE and the effect of the reaction between the two components on the phase decomposition behaviors was also investigated. We show the possibility of EFTEM for the investigation of the mechanism of reaction-induced phase separations.