Morphology and viscoelastic properties of a compatibilized epoxy/thermoplastic polymer blend were investigated. Transmission electron microscopy (TEM) showed that the introduction of the compatibilizer significantly reduces the size of the thermoplastic dispersed phase. Dynamic mechanical spectrometry (DMS) revealed an additional loss peak at about 333 K at 1 Hz in the compatibilized ternary blend spectrum which is detailed as the first unequivocal observation of a 'micromechanical transition' in a compatibilized thermoset/thermoplastic blend. In addition, the influence of thermal treatments (which were established according to structural recovery considerations, i.e. accelerated physical aging at T-g - 20 K and 'refreshing' process at T-g + 20 K) on the viscoelastic behavior (amplitude and temperature position) of the additional loss peak was studied. These investigations revealed that the occurrence of the 'micromechanical transition' on DMS spectra depends significantly on the thermal history of the sample and leads to the assumption that the viscoelastic behavior of the interphase relaxation is associated with the conditions of confinement of the components in the interphase zone.