The phase-inversion phenomena of diglycidyl ether of bisphenol A (DGEBA) incorporated with a carboxy-terminated butadiene-acrylonitrile (CTBN) containing 17 wt % bound acrylonitrile after curing with ethylene diamine (EDA) in a stoichiometric ratio were investigated by scanning and transmission electron microscopy (SEM and TEM), and wide-angle X-ray scattering (WAXS). The results of the SEM studies revealed that the rubber-rich phases in a size range of 1-30 mum, which originally dispersed in an epoxy-rich matrix, gradually transformed to a co-continuous phase as the CTBN content reached 30 phr. As the CTBN content was increased to 50 phr, that phase inversion, as the continuous epoxy-rich phase transformed to a dispersed phase, took place. However, because the transformed dispersed phase was very small, the phase inversion could only be observed by TEM in specimens stained by osmium tetraoxide (OsO4). On the other hand, we also found that the phase inversion could be detected by WAXS, and this showed the absence of a characterization peak of the epoxy resin at the scattering vector, Q=4.1 nm-1. To the best of our knowledge, this was the first time in the literature that WAXS has been employed to detect a phase inversion.