The viscoelastic properties of a compatibilized epoxy/thermoplastic polymer blend were investigated. Mechanical spectrometry revealed an additional loss peak at about 333 K at 1 Hz in the compatibilized ternary blend spectrum that is detailed as a “micromechanical transition.” Such an interphase relaxation does not correspond to a relaxation of one of the pure components. Mechanical modeling was used in a reverse mode to investigate the actual characteristics of the interphase (i.e., volume content and mechanical properties).