Operating characteristics, including capacitance, leakage current, operating potential range, cycling stability and open- circuit self- discharge behaviours, of the magnetite ( Fe3O4) supercapacitor, containing 10 wt % carbon black as conductive additive, in aqueous electrolytes of Na2SO3, KOH and Na2SO4 were investigated. Although the capacitance of the oxide was found to depend heavily on electrolyte composition, the self- discharge mechanism in these electrolytes appeared to be the same. Reduction in the dissolved oxygen content ( DOC) of the electrolyte reduced the leakage current and profoundly improved the cycling stability. In particular, Na2SO3(aq) gives the highest capacitance, nearly 30 F (g- Fe3O4)(-1) or 80 muF cm(-2) of actual surface area, with an operation range of 1.1 V based on a leakage current less than 0.1 mA F-1, and the electrode showed no deterioration after 10(4) cycles under a DOC < 0.1 ppm.