The effect of two poly(styrene-graft-ethylene oxide)s with different graft lengths and graft densities on the interphase properties of poly(2,6-dimethyl-1,4-phenylene oxide)/poly(methyl methacrylate) (PPO/PMMA) blends was studied by different techniques. The miscibility between the copolymers and the homopolymers, as well as the phase reduction in the ternary blends, was favoured by longer and more separated grafts. Transmission electron microscopy shows that the addition of any of the copolymers leads to the formation of an interphase between the heterogeneous PPO and PMMA phases. The ternary blends present micromechanical and microdielectric transitions at temperatures in the range 60-100 degrees C, where the transition temperatures decrease with increasing copolymer contents. The micromechanical and microdielectric transitions originate from the change in the relative moduli values and the dielectric losses of the matrix, interphase and particle phase of the ternary blends.