The palladium(II)-substituted tungstosilicate [CS2K(H2O)7Pd(2)WO(H2O)(A-alpha-SiW9O34)(2)](9-) (1) has been synthesized and characterized by IR, elemental analysis, and electrochemistry. Single-crystal X-ray analysis was carried out on CS3K2Na4[Cs2K(H2O)(7)Pd2WO(H2O)(A-alpha-SiW9O34)(2)].5H(2)O (1a), which crystallizes in the monoclinic system, space group P2(1)/n, with a = 16.655(3) Angstrom, b = 19.729(4) Angstrom, beta = 25.995(5) Angstrom,beta = 95.46(3)degrees, and Z = 4. Polyanion 1 represents the first structurally characterized palladium(II)-substituted tungstosilicate. The title polyanion consists of two (A-a-SiW9O34) Keggin moieties linked via a {WO(H2O)(4+) group and two equivalent, square-planar Pd2+ ions leading to a sandwich-type structure with C-2v symmetry. The central belt of 1 contains also one potassium and two cesium ions. Polyanion 1 was synthesized by reaction of Pd(CH3COO)(2) with K-10[A-alpha-SiW9O34] in aqueous acidic medium (pH 4.8). A cyclic voltammetry study of polyanion 1 in a pH 5 medium shows a Pd-0 deposition process on the glassy carbon electrode surface. The corresponding wave and that of tungsten redox processes could be separated clearly during the first few runs before their merging into a broad composite wave. The film thickness increases with the number of potential cycles or the duration of potentiostatic electrolysis. As judged from hydrogen sorption/desorption pattern, the quality of the film deposited from polyanion 1 is better than that of a film deposited directly from Pd2+ solutions.