The plasticization effects of cellulose diacetate composite systems including nanoparticles (montmorillonite, MMT) and plasticizers(diethyl phthalate, DEP) were investigated by the time-temperature superposition technique and viscoelastic modeling. Exhibiting the highest modulus value in the glass state, the viscoelastic modulus of the MMT nanocomposite rapidly decreased above the glass-transition temperature (T-g). The Arrhenius-type activation energy of pristine cellulose acetate showed the lowest value of activation energy and both DEP-plasticized and MMT-reinforced systems exhibited increased values of activation energy. Although the free volume fraction at the T. decreased with the plasticizer content, it increased with the incorporation of MMT, seemingly preventing the polymer chains from being arranged in an ordered structure. (C) 2004 Wiley Periodicals, Inc.