Water transport through nanocomposite-based polyamide 12/montmorillonite was investigated by permeation kinetics. Intercalated and semi-exfoliated nanocomposite structures were studied. For low montmorillonite content (<= 5%) a decrease of water permeability was obtained for a both structures and mainly explained by an increase of the tortuosity (decrease of water diffusivity). However, this barrier effect is affected by other phenomena which depend on the structures: the plasticization phenomenon, the water solubility and the role played by the clay/polymer interface.