The objective of this study was to understand the chemical modification impact on interactions between water and cellulosic fiber. In that respect, cotton (C), flax (F), viscose (V) and cellulose acetate (CA) were analyzed by using a dynamic vapor sorption analysis. The sorption and desorption isotherms and kinetic curves were modelled using the Park model and the “Parallel Exponential Kinetics” (PEK) model-which allowed an accurate fitting on the whole range of water activity. The obtained sorption properties were correlated to the accessibility and the amount of sorption sites and also to the crystallinity level of the fibers. It was found that V exhibited the highest water sorption capacity due to a higher hydroxyl groups accessibility and a high amorphous fraction, followed up by F, C and CA. In contrast, higher kinetic sorption rate was obtained for CA due to a decrease of the hydroxyl groups within the fibers. Regardless the fiber, the determination of characteristic times showed that the kinetic rate was higher for sorption than desorption.