AbstractMorphology evolution in ternary polymer solutions consisting of poly(methyl methacrylate), poly(styrene‐co‐acrylonitrile) having different AN‐contents and toluene was studied by optical microscopy and light scattering. The solutions subjected to different isothermal demixing conditions displayed regularly or irregularly phase separated structures. Effects of the AN‐content of the SAN, the blending ratio and the annealing conditions on the arising morphologies are presented. A criterion for conservation of the periodic bicontinuous structure developed in the unmixing process can be esta