The aim of this work is to study the effect of an organically modified clay on the structural relaxation behavior of an amorphous thermoplastic matrix. Differential scanning calorimetry (DSC) and pressure-volume-temperature (PVT) analysis were used in order to calculate the relaxation enthalpy and specific volume of the neat matrix and the nanocomposite during ageing below the glass transition region. Stress relaxation experiments were used to measure the characteristic relaxation time of the materials at different temperatures below glass transition. DSC and PVT analysis both revealed that relaxation phenomena are more relevant for the nanocomposite compared to the neat matrix. This result was confirmed by stress relaxation measurements, which indicates that below the glass transition the relaxation times of the nanocomposite are lower than those of the neat matrix. The enhanced relaxation behavior of the nanocomposite compared to the neat matrix was attributed to the presence of the organic modifier, which, acting as a plasticizer for the matrix, enhances its molecular mobility. The presence of low glass transition portions in the matrix was also evidenced by DSC and dynamic mechanical analysis. (C) 2012 Elsevier B.V. All rights reserved.