Monohydrated single crystals of dioctadecyldimethylammonium chloride (DC(18)DA(+) Cl-.H2O) were prepared and the crystal structure was determined to elucidate the effects of water molecules on the crystalline to liquid-crystalline phase transition behavior. From the observation during the phase transition of as-prepared single crystals by polarizing optical microscopy and powder X-ray diffractometry, we demonstrated that the water molecules are firstly melted with increasing temperature and one-dimensional ionic chains, [-{Cl-2(H2O)(2)}(2-)-{(N)(2)}(2+)-](n), are merged into two-dimensional ionic layers by sliding on the (1 1 -5) planes. This rearrangement is accompanied by the formation of a smectic A like bilayer structure. In addition, the one-dimensional ionic chains are simultaneously stretched to the chain direction, and alkyl chains are interdigitated to reduce a free space within the hydrophobic unit.