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Single-crystalline nanowires of hexagonal γ-MnO₂ phase have been prepared by a simple method in which the commercial granular γ-MnO₂ powders were hydrothermally treated in water or ammonia solution at 120–160 °C. The diameters of the γ-MnO₂ nanowires are 5–50 nm and the lengths up to several tens of micrometers. Calcination of γ-MnO₂ nanowires at the temperature range of 400–500 °C and above 600 °C results in, respectively, the production of single phases of β-MnO₂ and α-Mn₂O₃ crystals, while the nanofibrous morphologies persist. These 1-D nanostructured manganese oxides are expected to have possible novel properties and wide applications.