A fluorinated polyimide was employed to investigate reaction-induced phase separation of epoxy/polyimide blends. Three distinct morphological structures were obtained. Among these, an unusual three-layered structure was observed in a certain polyimide concentration range. The polyimide-rich phase was mainly localized in the middle layer and the outer layers were epoxy-rich. The thickness of the Outer layers (10-30 mu m) was Much larger than the characteristic length of the bicontinous domains in the middle layer. In the lower polyimide concentration range. polyimide-rich particles formed in the epoxy-rich matrix its aggregates without further coalescence. At higher polyimide concentrations, polyimide-rich phase formed continuous domains while epoxy-rich phase formed connected nonspherical particles. Evolution of the layered Structure was elucidated from scanning electron micrographs. No layer existed in the sample before phase separation began. The reaction conversion of epoxy was very low when layers started to form. During that period, the Outer layers thickened rapidly. The mechanism of this unusual layered structure formation Was discussed based on the characteristics of this epoxy/polyimide system. Also a refractive index cross-over was observed which was caused by the gradual increase of the refractive index of epoxy during the isothermal Curing process.