Dynamic water vapor sorption on lyocell and cotton fibers was gravimetrically investigated at 20 °C. Lyocell fiber showed a higher equilibrium moisture regain of 9.23%w/w at 60% relative humidity compared to 5.54%w/w for the cotton. The hysteresis between the sorption and desorption isotherms for lyocell was 46.9% and higher than 24.7% for cotton. The hysteresis decreased with increasing relative humidity of the atmosphere. A good fit of the experimental data with the parallel exponential kinetics model suggests that moisture exchange on lyocell and on cotton is based on two different mechanisms. The kinetic parameters for the identified components of slow and fast sorption were estimated from the simulations and moreover the BET surface volume was calculated. The mechanism of water vapor sorption on cellulosic fibers are discussed considering the effects of the relative humidity on the kinetic parameters, water retention capacity, BET surface volume and the hysteresis effect. © 2004 Elsevier Ltd. All rights reserved.