MnFe2O4-carbon black (CB) composite powders synthesized by a co-precipitation method have been characterized and optimized for their electrochemical properties for supercapacitor applications. The composite shows pseudocapacitance in electrolyte solutions of alkali and alkaline chlorides, sulfates and sulfites. For the chlorides and sulfates electrolytes, the pseudocapacitance has been identified, by in situ X-ray absorption near-edge spectroscopy study, to involve charge-transfer at both the Mn and Fe sites of the ferrite. In 1 M NaCl(aq), the composite electrode exhibits an operating potential window of 1.0 V with a maximum leakage current of 0.3 mA F-1, and it exhibits far superior cycling stability to amorphous MnO2 electrode. Both the specific capacitance and self-discharge behavior of the composite electrode depend strongly on the composite composition. The optimum capacitance occurs at ferrite:CB weight ratio of 7:3, which gives reduced self-discharge rate as compared with CB. The composite electrode also demonstrates capability of high-power delivery. (c) 2006 Elsevier B.V. All rights reserved.