Interaction of the trilacunary Keggin polyanions [A-alpha-XW9O34](10-) (X = Si-IV, Ge-IV) with Co-II and phosphate ions in aqueous, basic media and under mild heating leads to the formation of the tetrameric, Co-16-containing heteropolytungstates [{Co-4(OH)(3)PO4}(4)(A-alpha-XW9O34)(4)](32-) (X = Si-IV, Ge-IV). Both polyanions were characterized in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravirnetric and elemental analyses. Furthermore, the electrochemical and magnetic properties of these isostructural polyanions were investigated.