Hydrothermal decomposition of permanganate, conducted in a range of pH-controlled solutions (from strongly acidic to strongly basic), is used to prepare manganese dioxides that are well-suited for use as supercapacitor electrode materials. While permanganate is thermodynamically unstable, the kinetics of its decomposition in an aqueous environment are very slow, until the temperature is raised to similar to 200 degrees C. Although the resultant materials are relatively crystalline and have lowtotal pore volume, their prominent meso-porosity leads to good electrochemical performance. Best behaviour is obtained for material from permanganate decomposition in 0.01M H2SO4 solution, for which composite electrodes (150 mu m thick) yield similar to 150F g(-1) at 5mV s(-1) in a 9M KOH electrolyte. Crown Copyright (C) 2009 Published by Elsevier B. V. All rights reserved.