Amorphous cellulose exhibits two mechanical relaxation processes, so-called gamma and beta, below its glass transition temperature T-g. Though much work has already been published on the subject, the origin of these relaxations is still uncertain, especially because most of the dynamic mechanical data were obtained at constant frequency and mixed together with dielectric data. In order to reach more definitive answers, two main approaches were taken in this work, namely (i) the use of a low-frequency mechanical spectroscopic technique and (ii) the comparison with data of different polysaccharides, having different lateral groups and different intramolecular links along the main chain. In addition, preliminary molecular modeling based on molecular mechanics was performed on isolated chains in vacuum. The work is focused on dried cellulose, to minimize the effect of relaxation overlap, which becomes negligible for moisture content lower than 2%. The results show that the gamma process occurs without cooperativity and confirm its origin in the rotation of primary hydroxyl groups. In fact, in contrast with the beta relaxation, its activation energy does not involve significant entropic contribution, and molecular modeling suggests that the rotation of those lateral groups does not lead to conformational change in the rest of the chain (no cooperativity). The activation energy of the beta relaxation involves an entropy contribution that varies with the water content.