Novel types of water-soluble anionic cubic cages, K-4[{Co-III(Me-3-Tacn)}(4) {Fe-II(CN)(6)}(4)] and Na-4 [{Co-III(Me-3-Tacn)}(4){Fe-II(CN)(6)}(4)] (Me-3-Tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), were prepared by means of a mechanistically designed self-assembly process between [Co(Me-3 -Tacn)Cl-3] and A(4)[Fe-II(CN)(6)] (M = Na or K), consisting of a rate-limiting outer-sphere redox step, followed by a fast substitution/inner-sphere redox reaction sequence. These compounds show remarkable stability in aqueous solution at different pH ranges, displaying neat protonation processes and reversible oxidation with peroxodisulfate to its neutral {Fe-4(III) Co-4(III)} form. Furthermore, the cages behave as a robust and water-soluble molecular Prussian Blue analogue capable of encapsulating {Na-OH2}(+) pairs and K+ cations in aqueous solution, with the cubic structure of the complex being preserved. Substitution of the {Na-OH2}(+) pairs by K+ is easily accomplished, and the electrochemical properties of the sodium and potassium salts of the new cages have been found to be dramatically dependent on the encapsulated units.