Single crystal nanoflakes and nanorods of Ni(OH)(2), CO(OH)(2) and Fe3O4 were prepared by a controlled hydrothermal conversion route from MC(2)O(4)(.)2H(2)O in the NaOH solution. The structure and morphology of the final products and their oxalate precursors were characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The formation mechanism of conversion was suggested in this paper, accompanied the morphology control hypothesis. (C) 2002 Elsevier Science B.V. All rights reserved.