The dimeric, pentacopper(II) substituted tungstosilicate [Cu-5(OH)(4)-(H2O)2(A-alpha(-SiW9O33)2](10-) (1) has been synthesized in good yield using a one-pot procedure by reaction Of CU2+ ions with the trilacunary precursor salt K-10[A-alpha-SiW9O34]. The title polyanion represents the first polyoxotungstate substituted by 5 copper centers and the central copper-hydroxo-aqua fragment is completely unprecedented. In the course of the reaction, two [A-alpha-SiW9O34](10-) Keggin half-units have fused in an asymmetrical fashion resulting in the lacunary polyoxotungstate [Si(2)W18O(66)](16-) The vacancy in this species is stabilized by a magnetic cluster of five octahedrally coordinated CU2+ ions resulting in polyanion 1 with C-2v symmetry.