The effect of replacing water (H2O) with heavy water (D2O) on both lamellar and nonlamellar structures formed by phospholipids and monoolein was studied by X-ray diffraction methods. Except for lamellar crystal and inverted hexagonal phases, the lattice constants of all phases investigated were decreased. The changes in the cubic phases of monoolein can be interpreted as a reduction in the interfacial area caused by D2O. The largest change was observed in the Im3m bicontinuous cubic phase of monoolein.