Polymer nanocomposites based on an organophilically modified montmorillonite (OMMT) and polystyrene ( PS) are prepared via a solvent casting method using chloroform as a cosolvent to examine their dynamic viscoelastic and relaxation properties, in which the increased basal spacings of the OMMT determined by X-ray diffraction indicated the intercalation of PS chains into OMMT interlayer. From the measured viscoelastic properties originated from the nanocomposite interaction between polymer and OMMT, we were able to determine the characteristic behavior of PS/OMMT nanocomposites. Storage and loss moduli are found to give the transition from liquid-like to solid-like behavior as the clay content increases, especially in the low frequency region. Stress relaxation behavior was also enhanced by showing more solid-like characteristics with increasing OMMT. (C) 2003 Kluwer Academic Publishers.