Nanometer-scale particles of MnO2 have been synthesized by microemulsion route for electrochemical supercapacitor studies. The MnO2 has been found to be in alpha-cyrstallographic form with tetragonal unit cell. Particles in the spherical/hexagonal shape with about 50 nm size have been observed in scanning electron microscopy and transmission electron microscopy studies. Cyclic voltammograms have exhibited rectangular shape between 0 and 1.0 V vs saturated calomel electrode at sweep rates up to 100 mV s(-1) due to nanoparticles of MnO2. From galvanostatic charge-discharge studies, specific capacitance of 297 F g(-1) has been obtained, which is greater than about 240 F g(-1) usually reported for this material. The alpha-MnO2 samples have been annealed at several temperatures, and nanoparticles (10 - 90 nm) and nanorods (5 nm diameter) of varying dimensions have been obtained. The effect of annealing at different temperatures on crystallographic nature and electrochemical properties are reported. (c) 2006 The Electrochemical Society.