Processing of intractable polymers using reactive solvents (monomers) has been studied extensively in our laboratory, notably the system poly(phenylene ether) (PPE)/epoxy (resin). PPE can be dissolved at elevated temperatures in epoxy resin and the solution can be easily transferred into a mould or into a fabric. Upon curing the epoxy resin, phase separation and phase inversion occurs and the originally dissolved PPE becomes the continuous matrix phase. The dispersed (cured thermoset) epoxy particles become an integrated part of the system and could act as fillers or as toughening agents, depending on the type of epoxy resin used. An important parameter for the (ultimate) physical and mechanical properties is the size of the dispersed particles. The aim of the present study is to control the morphology development in order to produce a dispersed phase in the sub-micron to nanometre range. The size of the dispersed phase will be determined by the competition between the coarsening rate, e.g. by the coalescence of dispersed droplets, and the vitrification and/or gelation rate induced by curing. For the coarsening process, the viscosity of the system plays an important role which is usually mainly determined by the temperature. However, in the case of PPE/epoxy, the viscosity can be controlled at a chosen curing temperature by adding polystyrene. The ternary phase diagram shows that the miscibility of PPE–polystyrene (PS) is retained upon the addition of epoxy at relatively low concentrations. However, thermally induced phase separation upon cooling occurs for solutions with an epoxy content of 30 wt% and more. Upon curing, a two phase morphology is obtained in which the PPE–PS phase acts, as expected, as one single phase. The size of the dispersed phase can be decreased by one order of magnitude if curing is performed at the glass transition temperature, Tg, of the initial solution, attributed to the high viscosity at Tg that slows down coalescence. During the additional post-curing steps, necessary to reach a maximum epoxy conversion, these original morphologies are maintained. In conclusion, by controlling the polymerisation temperature, relative with reference to the Tg of the original solution, the final morphology of the chemically induced phase separated systems can be tuned.