The reaction of the trivacant Dawson polyoxometalate alpha-[P2W15O56](12-) and the divalent cations Co2+ is known to form a symmetrically derived sandwich complex of formula beta beta-[Co-4(H2O)(2)(P2W15O56)(2)](16-) [symbolized as beta beta-Co-4(P2W15)(2)] at low pH (ca. pH 3). We have shown previously that, by a slight modification of the reaction conditions, trinuclear alpha beta-[(NaOH2)Co-3(H2O)(P2W15O56)(2)](17-) and dinuclear [(NaOH2)(2)Co-2(P2W15O56)(2)](18-) complexes [symbolized as alpha beta-NaCo3(P2W15)(2) and Na2Co2(P2W15)(2), respectively] can be synthesized as aqueous-soluble sodium salts. alpha beta-NaCo3(P2W15)(2) is a "lacunary" sandwich complex that can add a Co2+ cation to form nearly quantitatively an unsymmetrical Dawson tetracobalt sandwich polyoxometalate, alpha beta-[Co-4(H2O)(2)(P2W15O56)(2)](16-) [symbolized as alpha beta-Co-4(P2W15)(2)]. Thus, for Co-4(P2W15)(2), the junctions between the trivacant {P2W15} subunits and the central tetrameric unit can be either both beta type or beta and alpha types. The interconversion between alpha beta-Co-4(P2W15)(2) and beta beta-Co-4(P2W15)(2) and the decomplexation process at low pH, leading to the formation of alpha beta-NaCo3(P2W15)(2) and/or Na2Co2(P2W15)(2), have been followed in aqueous solution at various pH values by electrochemistry, UV-visible absorption spectroscopy, and P-31 NMR spectroscopy.