Broad-band dielectric relaxation measurements were performed for the four pentitols isomers, xylitol, adonitol, L-arabitol and D-arabitol. The comparison of the dynamical properties of these compounds shows similarities between the secondary relaxation processes but also important differences for the temperature dependence of the primary process characterized by the steepness index. These differences enable us to distinguish two groups of compounds that correspond to two kinds of molecular conformation. We show that the formation of more or less extended networks of hydrogen bonds, which reflects the more or less non-Arrhenius relaxation, can be related to the differences of conformation of the studied isomers. variation of the primary