The redox reaction of ferrocyanide was investigated for possible use as a redox additive for the prevention of the electrolyte decomposition of aqueous secondary Ni–MH batteries in the overcharged condition. It was found that with the presence of ferrocyanide, the charging voltage can be leveled off just above the complete oxidation of the positive nickel electrode. As a result, the oxygen evolution was greatly suppressed and the internal pressure of the batteries was kept at low level even at prolonged overcharging. In addition, no detrimental effects of the redox additive were observed on the normal charge–discharge performance of Ni–MH batteries.