AbstractThe double hydroxides Mn(OH)2Mg(OH)2and Mn(OH)2Ca(OH)2, precipitated by computer control, were investigated as positive electrodes for secondary cells in aqueous potassium hydroxide solutions with regard to charge capacity and cycling behaviour. Mn(OH)2Mg(OH)2‐hydroxides form solid solutions, whereas Mn(OH)2Ca(OH)2‐hydroxides occur as mixtures. In the system Mn(OH)2Mg(OH)2only samples between 30 and 60 mol% Mg are electrochemically active and discharge capacities of 164–213 mAh/g are found in the first discharge process, with an oxidation state corresponding to MnO1,8. Afterwards the capacity falls to 24 mAh/g. Surprising in the system Mn(OH)2Ca(OH)2is the rapid air oxidation of the samples with 30–50 mol% Ca to oxidation degrees of MnO1.8. These samples are further oxidized electrochemically to ∼MnO2.0, and are electrochemically active with discharging capacities between 96 and 133 mAh/g. They have a better cycling behaviour than the Mn/Mg‐species. The striking behaviour of the two double hydroxides is a consequence of their s