Manganese dioxide exhibits the ideal capacitive behavior in aqueous electrolytes containing alkaline-earth cations (Mg2+, Ca2+, or Ba2+). A specific capacitance value as high as 325 F g(-1) was obtained in 0.1 mol L-1 Mg(NO3)(2) electrolyte at 2 mV s(-1). The ideal capacitive behavior, high specific capacitance, good coulombic efficiency, and rate ability of manganese dioxides in aqueous electrolytes containing bivalent cations (Mg2+, Ca2+, or Ba2+) indicate that these alkaline-earth cations might be good alternatives for the state-of-the-art univalent alkaline cations. Moreover, activated carbon/MnO2 asymmetric capacitors with 2 V operating voltage were built up based on the aqueous electrolytes containing these alkaline-earth cations. The energy density of the AC/MnO2 asymmetric capacitor with Ca2+ cation at a current density of 0.3 A g(-1) was found to be 21 Wh Kg(-1).