A basic concept is discussed for bulk solid-solid conversions of metal oxide electrodes via electron/proton transfer in aqueous electrolytes at room temperature. Consequences of this model are investigated using binary and ternary copper oxides as examples. It is demonstrated that reactions of this type may proceed with quantitative conversion of the metal oxide under formation of a metastable and highly reactive microporous metal matrix in a pseudomorphous transition mode. With appropriate starting oxide phases it is possible to obtain metastable alloys and heterogeneous systems via low temperature solid state reactions.