The palladium-substituted tungstoantimonate(III) [Cs2Na(H2O)(10)Pd-3(alpha-SbW9O33)(2)](9-) (1) has been synthesized and characterized by IR, elemental analysis, and electrochemistry. Single-crystal X-ray analysis was carried out on Cs3KNa5[Cs2Na(H2O)(10)Pd-3(SbW9O33)(2)].16.5H(2)O, which crystallizes in the monoclinic system, space group P2(1)/m, with a = 13.3963(13) Angstrom, b = 19.5970(19) Angstrom, c = 18.1723(17) Angstrom, beta = 100.416(2)degrees, and Z = 2. Polyanion 1 represents the first structurally characterized palladium(II)-substituted polyoxometalate. The title polyoxoanion consists of two (alpha-(SbW9O33)-W-III) Keggin moieties linked via three Pd2+ ions leading to a sandwich-type structure. The palladium centers are equivalent, and they are coordinated in a square-planar fashion. The central belt of 1 contains also one sodium and two cesium ions which reduces the symmetry of the polyanion to C-2v. Polyanion 1 was synthesized in good yield by reaction of Pd(CH3COO)(2) with Na-9[SbW9O33] in aqueous acidic medium (pH 4.8). A cyclic voltammetry study of polyanion 1 in a pH 5 medium gives essentially the same characteristics as those observed for the deposition of Pd-0 on the glassy carbon electrode surface from Pd2+ solutions. The film thickness increases with the number of potential cycles or the duration of potentiostatic electrolysis. The particularly sharp hydrogen sorption/desorption pattern indicates the excellent quality of the Pd-0 deposit from polyanion 1.