Because of their good mechanical properties and low density, natural fibers are more and more considered as reinforcement in composite materials. To improve the adhesion between the polymer matrix and natural fibers and to reduce the water sorption, helium cold plasma and autoclave treatments have been performed on flax fibers. The effect of these treatments on water sorption has been investigated by means of a gravimetric static equilibrium method (IGA). Water sorption isotherms have been deduced from kinetic data. Specific mathematical models have been tested to predict the water sorption isotherms of the treated flax fibers. The Park model based on the three sorption modes: Langmuir, Henry's law, and clustering, was successfully used to simulate the experimental sorption data. The results show no significant effect after plasma treatment while an increase of moisture resistance has been observed after autoclave treatment. This effect of autoclave treatment is interpreted in terms of modifications of cell-wall structure. In a kinetic point of view, for the untreated and treated flax fibers, the variation of the diffusion coefficient with water concentration is in agreement with the three sorption modes of the Park model. (c) 2006 Wiley Periodicals, Inc.