The influence of a poly(styrene-graft-ethylene oxide) (P(S-g-EO)) copolymer on poly(2,6-dimethyl-p-phenylene oxide)/poly(methyl methacrylate) (PPO/PMMA) blends was studied by a number of different techniques in order to characterize the properties of the interphase. Scanning electron microscopy shows that the copolymer reduces the dispersed phase size, and dynamic mechanical spectroscopy (DMS) reveals a new micromechanical transition in the ternary blends. These effects are shown to be caused by a copolymer-rich interphase with a certain volume fraction and with its own characteristic properties. Solid-state nuclear magnetic resonance (NMR) relaxation times indicate that the poly(ethylene oxide) side chains of the copolymer are partially miscible with PMMA in the ternary blends, which suggests that the interphase does not only consist of pure copolymer.