The stiffness of PA6 silicate nanocomposites has been measured for a number of silicate fractions and moisture levels. The matrix modulus was varied experimentally by moisture conditioning, resulting in three different moisture levels. The moduli have been analysed with a Halpin-Tsai composite model. This model was used to calculate the matrix modulus in the nanocomposites based on the crystallinity, the amount of surfactant and the moisture content. The measured modulus of the nanocomposites with different moisture content was used to calculate the effective aspect ratios of the reinforcing particles with the Halpin-Tsai model. The calculated aspect ratio for each nanocomposite is independent on the moisture concentration. This indicates the validity of this composite model to describe nanocomposites, despite the small particle size. The results suggest that changes on the molecular level due to the presence of the silicate layers only have a small influence on the modulus, because the modulus can be explained with a composite model. The reduced efficiency of reinforcement at increasing filler levels can be partially explained by the reduction of the matrix modulus due to surfactant and reduced crystallinity, and partially by the occurrence of small stacks of platelets. (c) 2005 Elsevier Ltd. All rights reserved.