Cyclic voltammetry was carried out on selected vanadium-substituted Keggin polyoxometalates in aqueous solution with a focus on K3H2[alpha-SiVW11O40] center dot A 6H(2)O (SiVW11O40) and K4H2[gamma(1, 2)-SiV2W10O40] center dot A 4H(2)O (SiV2W10O40). The redox waves of the V-O and W-O framework for both polyanions are pH- and scan rate-dependent. The first vanadium-centered wave (V-wave) for SiVW11O40 shows a classical potential shift as a function of acidity at pH < 3.5, and then becomes pH-independent above this value. Above pH 3.5, the W-centers in SiVW11O40 and SiV2W10O40 exhibit efficient electrocatalytic activity towards the reduction of nitrite and hydrogen peroxide while the V-waves also exhibit an electrocatalytic response in nitrite-containing solutions.