Di- and tricobalt silicotungstate complexes, K5Na4H4[{Na-3(mu-OH2)(2)Co-2(mu-OH)(4)} (Si2W18O66)]center dot 37H(2)O (1) and K6Na3[Na(H2O){Co(H2O)(3)}(2){Co(H2O)(2)}(Si2W18O66)]center dot 22H(2)O (2), have been synthesized through reaction of cobalt chloride and [A-alpha-SiW9O34](10-) in acidic buffer solution. They have been characterized by X-ray crystallography, elemental analysis, cyclic voltammetry, infrared, and UV-vis spectroscopy. In 1, two cobalt atoms as well as three sodium atoms are incorporated in the central pocket of the [Si2W18O66](16-) polyanion. In 2, one cobalt atom and one sodium atom are incorporated in the pocket of [Si2W18O66](16-); two other cobalt atoms in this complex protrude outside the pocket and connect with WO6 units of other [Si2W18O66](16-) polyanions to form a one-dimensional polymeric structure. The crucial parameters in the synthesis of these two compounds are discussed, and their stability in different buffer solutions is studied. The decomposition of 1 or 2 in heated potassium acetate buffer (pH 4.8, 1 M) yields K-11[{Co-2(H2O)(8)}K(Si2W18O66)]center dot 17H(2)O (3) based on spectroscopic studies and an X-ray crystal structure.