Abstract All of the likely conformations of cellobiose, cellulose, and xylan have been explored systematically by using an electronic computer, assuming the ring conformations and (C-1)-O-(C-4′) angle for each pair of residues to be fixed and derivable from known crystal structures. The absolute van der Waals energies, but not the relative energies of different conformations, are sensitive to the choice of energy functions and atomic coordinates. The conformation of cellobiose in the crystal occurs near the minimum for intramolecular van der Waals interactions, perhaps slightly displaced to allow formation of an inter-residue hydrogen bond. The Hermans chain conformation for cellulose corresponds to a somewhat higher van der Waals energy, but this is probably offset by efficient crystal packing resulting from the two fold screw axis, and the hydrogen bond remains. The Meyer and Misch conformation is unlikely, because of its high van der Waals energy and for other reasons. The results lead to possible explanations of the known conformational stiffness of cellulose and its solubility properties in alkali. The characteristics of xylan conformations have been compared with cellulose.