The phase-separation behavior and morphologies of a commercial epoxy resin based on diglycidyl ether of bisphenol A cured with the diamine hardener 1,3-bisaminomethylcyclohexane and blended with different amounts of acrylonitrile-butadiene-styrene thermoplastic were studied with dynamic mechanical analysis and other techniques. In comparison with the neat system, the modified systems reached before gelation and vitrification, events that were assigned to the onset and endset of the storage modulus signals in a dynamic mechanical analysis curve (storage modulus versus time). These results were explained by a phase-separation phenomenon induced by polymerization. Scanning electron microscopy and light transmission focused on phase separation revealed a sea-island morphology for low thermoplastic contents, a bicontinuous structure for intermediate thermoplastic contents, and an epoxy-nodular phase-inverted structure for high thermoplastic contents.