A poly(styrene-graft-ethylene oxide) (P(S-g-EO)) graft copolymer affects the dynamic mechanical properties of a random poly(styrene-co-acrylonitrile) (SAN) copolymer filled with barium sulphate (BaSO4) through the location of the graft copolymer as an interphase between the filler and the SAN matrix. The addition of the graft copolymer results in a new transition, positioned at a lower temperature than the glass transition of SAN. The new transition is caused by the temperature-dependent change in relative dynamic mechanical properties of the components in the filled polymer blend and is called a micromechanical transition. It is shown that the new transition does not originate from a molecular transition in any of the constituents. The measured properties are compared with calculations using an interlayer model. The effects of the interlayer increase with the thickness of the interphase. The dynamic shear modulus decreases, and the micromechanical transition is positioned at a lower temperature as the concentration of the graft copolymer is increased.