The effect of compatibilization with a symmetrical polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM) triblock terpolymer on the morphological and mechanical properties (specifically toughness) of immiscible poly(2,6-dimethyl-1,4-phenylene ether)/poly(styrene-co-acrylonitrile) (PPE/SAN) blends with different blend (w/w) ratios is investigated. We study the effect of blend viscosity on the localization of the compatibilizer at the blend interface, influencing the mechanical properties of the macroscopic material. The impact of the specific morphology of the blends, known as "raspberry morphology", on the final material will be explained using thermomechanical analysis and revealing relevant deformation mechanisms. The correlations between domain size, toughness of the blends and the corresponding toughening mechanisms are explained. After compatibilization, the fracture toughness is increased due to better bonding and stress transfer between the phases and smaller domain sizes with strong interface impose shear yielding of the matrix. Very small inter-domain distances hinder matrix deformations, which forces debonding as a "weaker" main deformation mechanism. It is found that a PPE/SAN ratio of 60/40 (w/w) has the optimum viscosity during processing to control the morphology. At this blend ratio the dispersed PPE domains have a low packing density, allowing shear deformations of the matrix. (C) 2015 Elsevier Ltd. All rights reserved.