The double-phase morphology of 5 wt% high molecular weight poly(methyl methacrylate) (PMMA) modified epoxy system was investigated by optical and scanning electron microscopic (SEM) techniques. PMMA-epoxy blend cured at 100 degrees C revealed that a bicontinuous secondary phase separation was observed in both epoxy and PMMA phases in the early stages of curing. Epoxy-rich particles were dispersed in the PMMA-rich phase, while PMMA-rich particles were segregated in the epoxy-rich phase, leading to double-phase bicontinuous morphology. The spinodal decomposition mechanism could probably be responsible for this secondary phase separation. From the SEM analysis, a morphology consisting of a rough striated continuous phase along with large smooth regions was observed. Rough striated domains are ascribed to the PMMA-rich phase and the smooth domains are assigned to the epoxy-rich phase, thus confirming the secondary phase separation. The PMMA-epoxy blend showed a slight increase in flexural properties and about 20% improvement in the fracture toughness.