A facile complex homogeneous precipitation method was successfully employed for the preparation of MnCO3 and Mn(OH)(2) crystals using hydrazine hydrate as the complexing agent in a hydrothermal system. The morphology of the products could be simply controlled by changing the reactant, the reaction temperature and the reaction time. X-ray powder diffraction results showed that the as-prepared MnCO3 and Mn(OH)(2) products could be indexed to the rhombohedral and hexagonal structure, respectively. The morphologies of the products were investigated by scanning electron microscopy, field-emission scanning electron microscopy, and transmission electron microscopy. It can be seen that the products showed various morphologies such as MnCO3 cocoons, prisms, spindles, anamorphic rhombohedra and Mn(OH)(2) quasi-hexagonal nanosheets, dumbbells, nanoflake bundles. (C) 2008 Elsevier B.V. All rights reserved.