Two novel high-nuclear copper-substituted polyoxometalates, Na-16[Cu-14(OH)(4)(H2O)(16)(SiW8O31)(4)]center dot 20.5H(2)O (1) and K10Na14[Cu-10(H2O)(2)(N-3)(4)(GeW9O34)(2)(GeW8O31)(2)]center dot 30H(2)O (2), containing 14 Cu2+ ions and 10 Cu2+ ions, respectively, have been obtained in aqueous solution and characterized by IR, UV, TG, element analysis, electrochemistry, and single-crystal X-ray analyses. The polyoxoanion framework of 1 is composed of four [beta-SiW8O31] units connected by 14 Cu2+ ions to constitute a tetrameric compound. In 2, two [beta-GeW8O31] anions and two [B-alpha-GeW9O34] anions are connected together by a [Cu-10(N-3)(4)O-32(H2O)(2)] {Cu-10(N-3)(4)} cluster to construct a novel tetrameric compound. The results of the electrocatalytic experiments reveal that the reduced species of 1 and 2 have electrocatalytic activities for nitrate reduction.