Magnesium hydroxide chloride hydrate (MHCH), Mg-2(OH)(3)Cl center dot 4H(2)O, nanowires have been synthesized via a facile hydrothermal approach using magnesium oxide and magnesium chloride as reactants. A series of experiments were conducted to define the key parameters, which determine the shape and phase of the nanowires. It was found that the concentration Of MgCl2, and the hydrothermal treatment conditions (temperature and time) are important for the control of the morphology evolution of Mg-2(OH)(3)Cl center dot 4H(2)O nanocrystals: at lower MgCl2 concentrations and lower temperatures (from room temperature to 100 degrees C) MHCH are prone to display needle-like morphology. With the further increase Of MgCl2 concentration and hydrothermal temperatures, the needle-like crystallites become the preferred nanowires morphology, with average diameter of about 20 nm. By carefully adjusting the molar ratio of MgO to MgCl2, the other two types of MHCH, i.e., Mg-3(OH)(5)Cl center dot 4H(2)O- and Mg-10(OH)(18)Cl-2 center dot 5H(2)O-phased nanowires can also be controlled synthesized. Our capability of obtaining MHCH nanowires not only benefits the wider applications based on MHCH as a brand new class of nanofiller materials in polymer composite engineering with very high aspect ratio, but also it serves a promising basis for systemically studying magnesium hydroxide carbonate hydrates and magnesium hydroxide sulfate hydrates nanostructures. (C) 2007 Elsevier B.V. All rights reserved.