The viscoelastic properties of compatibilized poly(2,6-dimethyl-p-phenylene oxide) and poly(methyl methacrylate) (PPO/PMMA) blends were studied by dynamic mechanical spectroscopy (d.m.s.) and the experimental data compared with an interlayer model. An interphase created by a poly(styrene-graft-ethylene oxide) (P(S-g-EO)) copolymer was found to significantly affect the dynamic mechanical behaviour of these blends, and it is shown that the addition of the copolymer results in a new transition in the blends. This is theoretically shown to be a ’micromechanical transition’, and is explained by the change in relative moduli values of the components in the matrix-interphase-particle structure of the blends, and not to originate from a molecular transition in any of the constituents. The micromechanical transition temperature is predicted to depend on the volume fraction and the Poisson ratio of the interphase and is affected by the influence of PPO and PMMA on the properties of the interphase.