CIRCULAR hydrogen bonds present a new, experimentally demonstrated principle showing how hydration water molecules and hydroxyl groups of macromolecules can cooperate to form a network-like pattern. Quantum chemical calculations show that chain-like H-bonds in the crystal lattice1 are energetically favoured above individual ones2,3. This is due to the cooperative effect, which leads to increased H-bonding activity of an OH-group if it is already accepting or donating an H-bond. These linear structures can close up to form circular arrangements comprising four and more OH-groups4–6. Such circles have actually been described for crystal structures of ice7 and of ice clathrates8, but in these cases the water molecules within the circles are related by crystallographic symmetry elements and are therefore not independent of each other. One should expect to find lattice-independent circular H-bonds in crystal structures of large O—H-rich molecules which co-crystallise with water of hydration, conditions which are satisfied by the cyclodextrin family. The smallest member, α-cyclodextrin (α-CD; cyclohexaamylose), consists of six α(1, 4)-linked glucose molecules and contains six primary and 12 secondary hydroxyl groups ((C6H10O5)6, molecular weight 973). From aqueous solution, α-CD crystallises as hexahydrate, α-CD·6H2O, and this complex, with a total of 120 hydroxyl groups (4 × 18 from α-CD and 4 × 12 from the six H2O) in one unit cell has been studied by X-ray and neutron diffraction methods (ref. 9, and Klar, Hingerty and W.S., unpublished). Two other complexes, a second modification of the hexahydrate (K. Lindner and W.S., unpublished) and α-CD·methanol·4H2O (ref. 10) have been investigated by X rays. As refinement in all cases is around R = 4% for data extending to 0.89 A resolution, hydrogen atom positions could be assigned. I discuss here results obtained from the X-ray/neutron study of α-CD·6H2O.