The present work describes the anisotropy and instability observed upon the formation of co-continuous phase morphologies in model polystyrene/polypropylene melt-extruded blends. Uncompatibilized and reactively compatibilized blends using amino-terminated polystyrene, PS-NH2, and inaleic anhydride grafted polypropylene, PP-MAh, reactive precursors were investigated. Differences in phase morphology are discussed based on the viscoelastic properties of the components used, the blend composition and, the type and content of the compatibilizer precursor employed. As expected, for the same polystyrene grade at a concentration in the blend below 20 wt%, a polypropylene matrix having a higher viscosity enables the formation of a more co-continuous phase morphology than a less viscous one, as quantified by solvent extraction. The co-continuous phase morphology developed was found to exhibit a highly elongated structure upon melt flow through the die of the extruder. Isotropic co-continuity, observed inside the barrel of extruder, was transformed into anisotropic phase co-continuity in the form of interconnected infinite strands of the minor phase highly oriented in the extrusion direction. When the blends were thermally annealed, a 50150 PS/PP co-continuous blend exhibits a substantial phase coarsening from micro- to millimeter scale without alteration of the phase co-continuity. The reactive compatibilization of the polypropylene and the polystyrene phases using 5 wt% PP-graft-PS, reactively in situ generated was able to significantly retard the phase evolution process. (c) 2007 Elsevier Ltd. All rights reserved.