Morphology and phase separation process in blends of a net-work-forming reactive polymer. poly(aminopropylmethylsiloxane) (PAMS), in a poly(methyl methacrylate) (PMMA)-modified epoxy system were studied using optical, epifluorescence and scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical thermal analysis (DMTA). The thermoset, system was bisphenol A diglycidyl ether (DGEBA) with different PMMA, percentages. 2-10% w/w. Phase separation and reaction advancement were monitored in situ. At the concentration studied. PMMA does not influence the kinetics of the curing process, but it strongly affects the reactive compatibilization between DGEBA and PAMS. The morphology obtained consists of a continuous thermoplastic-rich phase surrounding thermosetting connected polyhedral particles of 5-15 mum. This cocontinuous morphology is observed independently of the percentage of PMMA. Results show that the morphology is strongly influenced by the diffusion and viscosity conditions during reactive compatibilization and phase separation. An increase in PMMA content leads to a decrease in the thermosetting polyhedral particle size. In contrast. an increase in curing temperature leads to bigger sizes. The addition of thermoplastic polymers to initially nonhomogeneous reactive blends is a potential route for generating cocontinuous morphologies irrespective of the thermoplastic content.