The preparation of copper(II) formate cathodes, their discharge in magnesium seawater cells, and the discharge reactions are described. This soluble active material could be discharged at high voltages (1.4−1.0 V) with high efficiencies (80–95%) in magnesium cells, when polystyrene solutions were used as the binder to make the cathode plates. For single cells energy densities in the range 70–120 Whkg−1 (based on the dry state) were obtained. The Cu(HCOO)2/Mg cell would meet many applications at low or moderate discharge rates as a substitute for the AgCl/Mg cell.