Sorption kinetic data for the interaction of water vapor with flax (Linum usitatissimum L.) were analyzed used the parallel exponential kinetics (PEK) model, with excellent fits to the data being obtained. The PEK model is the sum of two exponential kinetics processes (fast and slow), which have characteristic times and moisture contents associated with them. The slow adsorption and desorption processes exhibited important differences in their characteristic times, although hysteresis in the moisture contents was found to be predominantly associated with the fast process. The kinetics was examined over a range of relative humidity (RH) values and at different temperatures, enabling the determination of activation energies for the adsorption and desorption kinetic processes throughout the hygroscopic range (from 5-95%, RH). (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 21662173, 2010