In this work, in order to improve the adhesion between a polyester matrix (unsaturated polyester resin) and flax fibres (Linum usitatissimum L.) and to increase their moisture resistance, chemical surface treatments have been used. These different treatments were performed with maleic anhydride (MA), acetic anhydride (Ac), silane (Si) and styrene (S). The modified flax fibres were characterized by means of infrared spectroscopy and surface energy analysis. The effect of these treatments on water sorption was investigated by using a gravimetric static equilibrium method. Water sorption isotherms were derived from kinetic data. The Park model based on the three sorption modes: Langmuir, Henry's law and clustering, was successfully used to simulate the experimental sorption data. It was found that the (Ac) and particularly (S) treatments reduced overall water uptake of flax fibres. We show that tensile modulus, breaking strength and breaking strain depend on the chemical treatment used. (C) 2009 Elsevier Ltd. All rights reserved.