The frequency and temperature dependence of molecular and micromechanical transitions were studied in polymer blends with an interphase. The viscoelastic properties of poly(2,6 dimethyl-p-phenylene oxide) (PPO) and poly(methyl methacrylate) (PMMA) blends that were compatibilized by a poly(styrene-graft-ethylene oxide) (P(S-g-EO)) copolymer were studied by dynamic mechanical spectroscopy (DMS) and the experimental data were compared with an interlayer model. The addition of the copolymer resulted in a micromechanical transition, and the relation between the volume fraction of interphase, the activation energy of the micromechanical transition, and the micromechanical transition temperature was studied. A qualitative agreement between experiments and theory was achieved. The quantitative difference was explained by partial mixing of PPO and/or PMMA with the copolymer in the interphase.