Cellulose and dextran are biosynthesized polysaccharides, made of glucose repeat units linked together by (beta 1 --> 4) and (alpha 1 --> 6) linkages, respectively. Furthermore, cellulose has two hydroxyl groups and one hydroxymethyl group per glucose ring, while dextran has three hydroxyl groups and no hydroxymethyl group. This work deals with the characterization of dielectric secondary relaxations of amorphous dextran and cellulose. Dextran exhibits two dielectric secondary relaxations referred to as gamma(ddex) and beta(ddex), while cellulose has only one very broad relaxation, gamma(dcell). The gamma(ddex) relaxation process has an average activation energy and a pre-exponential time tau(o) of 32 kJ mol(-1) and 5 x 10(-15) s respectively. This weakly cooperative relaxation process should be associated with the rotation of hydroxyl groups. The beta(ddex) relaxation has an average activation energy and a pre-exponential time tau(o) of 82 kJ mol(-1) and 10(-20) s respectively. This activation energy has both enthalpic and entropic contributions. The comparison with mechanical relaxation data indicates that beta(ddex) results mainly from the motions of main chain segments. The analysis of the two dielectric relaxations of dextran leads to the conclusion that gamma(dcell) could result from the overlap of two processes corresponding respectively to the rotation of hydroxyl groups and to the rotation of hydroxymethyl groups.