A mixture of Pd(OAc)(2) and TBA(4)[gamma-SiW10O34(H2O)(2)] (TBA-SiW10, TBA = [(n-C4H9)(4)N](+)) showed high catalytic activities for hydration of various kinds of structurally diverse nitriles including aromatic, aliphatic, heteroaromatic, and double bond-containing ones. For hydration of 3-cyanopyridine, the turnover frequency was 860 h(-1), and the turnover number reached up to 670. A dipalladium-substituted gamma-Keggin silicodecatungstate, [gamma-H2SiW10O36Pd2(OAc)(2)](4-) (I), was successfully synthesized by the reaction of [gamma-SiW10O34(H2O)(2)](4-) with Pd(OAc)(2) in a mixed solvent of acetone and water. The crystal structure of I was a monomeric, dipalladium-substituted, gamma-Keggin silicodecatungstate with bidentate acetate ligands. Compound I showed similar activities and selectivities to those of a simple mixture of Pd(OAc)(2) and TBA-SiW10. The kinetic, mechanistic, and density functional theory calculation studies show that the dipalladium site plays an important role in the present hydration, and the nucleophilic attack of a hydroxide or water to the nitrile carbon atom is included in the rate-determining step.