Single-crystalline alpha-MnO2 nanowires and beta-MnO2 microcrystals have been synthesized by a hydrothermal method. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and simultaneous thermogravimetry-mass spectrometry (TG-MS). The effects of varying the hydrothermal reaction times and the initial reactant concentrations on the morphology and crystalline structures of the final products have been investigated. It was found that at high reactant concentrations, the alpha-MnO2 nanowires formed initially became self-assembled into bundles and subsequently underwent a phase transformation to beta-MnO2 microcrystals on prolonging the hydrothermal treatment. At low reactant concentrations, however, beta-MnO2 microcrystals were formed even after short hydrothermal reaction times. A possible mechanism for the oriented attachment of the alpha-MnO2 nanowires and subsequent phase transformation to beta-MnO2 microcrystals has been suggested. (C) 2007 Elsevier B.V. All rights reserved.