Diss fibers, also called Ampelodesmos mauritanicus, are one of the least studied fibers in the literature. Indeed, the rare studies discussed have focused on mechanical and physicochemical properties. The present work deals with hygrothermal characterization and the effect of treatments on their behavior. So, untreated and treated Diss fibers with chemical treatments (alkaline, silane, and acetic acid) and heat treatment were characterized by absorption tests by water immersion and others by water vapor adsorption and desorption (Proumid-SPS). The results found have shown that these fibers presented a type II sorption isotherm with hysteresis where their saturation point could reach up to 40% of their dry mass. However, the treatments carried out seem to be able to reduce their hydrophilic nature especially after heat, silane, and acetic acid treatments. The adsorption kinetics, on the other hand, also seem to be positively influenced by these treatments, especially after the chemical treatments.