The water sorption of chitosan has been studied at 20 degreesC. Water transport is governed by a Fickian process for relative humidities lower than 0.4, and in that range of partial pressures, the diffusion coefficient is concentration-dependent. At a higher activity, anomalous diffusion is observed. The sorption isotherm is well described by the Guggenheim-Anderson-de Boer (GAB) model, and the clustering phenomenon observed at high relative pressures can be studied with the parameters of this model. The water permeability coefficient greatly increases with the relative pressure, and the water plasticization effect leads to a loss of the gas barrier properties under wet conditions.