The electrochemical properties of an iridium(IV)-substituted Dawson type heteropolytungstate anion have been studied in organic and aqueous solutions using cyclic voltammetry. In acetonitrile solution, this species exhibits four-step one-electron reduction processes, which are ascribed to the W(VI --> V) processes of the heteropolyanion framework. The iridium center plays an important role in these reduction processes, though its redox waves are not observed in acetonitrile solution. In acid buffer solution, the iridium-substituted derivative also exhibits four couples of reversible redox waves. The first one-electron couple at more the positive potential is assigned to the redox process of the Ir-IV active center. The diffusion coefficient for the Ir-IV/Ir-III couple in pH 3.0 buffer solution is evaluated, D-0 = 2.1 x 10(-6) cm(2) s(-1). The other one-electron and two two-electron couples are associated with the redox processes of the tungsten-oxo framework. All these waves are pH dependent. The iridium center has an excellent catalytic effect on the reduction of nitrite anion in acid solution. The lacunary polyanion, alpha(2)-[P2W17O61](10-), exhibits no catalytic effects under the same conditions. The maximum rate constant of the overall electrocatalytic reaction is measured to be 4.0 x 10(3) M-1 s(-1) at pH 3.0. The kinetic data point to HNO2 as the reactive form of nitrite in the catalytic reaction. The catalytic mechanism is also discussed.