The temperature-induced phase transition of native cellulose was studied by X-ray diffraction and molecular dynamics (MD) simulation. Upon heating, this transition is characterized by an important expansion of the distance between the planes of glucopyranose rings, which is observed both experimentally and in MD. Computed trajectories suggest that this expansion is caused by a rotation of the exocyclic hydroxymethyl groups. Upon cooling, the phase transition, experimentally known as reversible, was found to be irreversible in the MD simulation when using current GROMOS 53a6 force field parameters. By varying one of these, related to the potential energy of the hydroxymethyl conformers, a reversible phase transition could be observed in silica. From the linear dependence of the transition temperature on the dihedral energy of the different conformers, the entropy change due to the phase transition could be estimated to be about 26 JK(-1) mol(-1). This value essentially reflects the additive contribution of the torsional entropies of the exocyclic moieties, as other conformational parameters appeared to have little effect on the phase transition.