The synthesis and characterization of (Bu(4)N)(4)Na-2[(1,5-COD)Ir.SiW9Nb3O40], a Keggin polyoxoanion-supported Ir(1,5-COD)(+) complex, is reported (1,5-COD = 1,5-cyclooctadiene). The complex was characterized by means of an elemental analysis (all elements), ultracentrifugation solution molecular-weight measurements, and H-1, C-13, and W-183 NMR and IR spectroscopies in both weakly coordinating and strongly coordinating solvents. The solvent-dependent NMR results demonstrate that, in weakly coordinating solvents, a C-s symmetry complex obtains. On the basis of literature precedent, two most plausible, slightly different C-s symmetry structures are proposed, one involving Ir(1,5-COD)(+) bonding to two ONbW bridging oxygens and one OW2 bridging oxygen and another involving the same two ONbW bridging oxygens but then one ONb terminal oxygen. The significance of this work is 3-fold : it describes the first Keggin-type polyoxoanion-supported Ir(1,5-COD)(+) complex; it describes the first Keggin anion:supported complex that also serves as an effective catalyst precursor; and it sets the stage for the first detailed comparison of the catalytic properties of two different polyoxoanion-supported catalysts of any type, namely the present Keggin anion-based complex (Bu(4)N)(4)Na-2 [(1,5- COD) Ir.SiW9Nb3O40] and the Wells-Dawson anion-based complex (Bu(4)N)(5)Na-3[(1,5-COD)Ir.P2W15Nb3O62] we reported previously.