Redox electrochemistry of Keggin type iridium-substituted heteropolytungstate anions, [(H2OIrXW11O39)-X-IV](n-) (X = B, n = 5; X = Si, Ge, n = 4; X = P, n = 3), have been investigated in acetonitrile and aqueous solutions using cyclic voltammetry. In acetonitrile solutions, these heteropolyanions exhibit two-step one-electron reduction processes. These redox waves are ascribed to the W(VI --> V) processes of the heteropolytungstate framework. The slope of a plot of standard redox potentials against the ionic charge is evaluated. In acid buffer solutions (pH 1.0 similar to 6.0), all these anions exhibit two pairs of reversible, one-electron redox peaks. One pair is assigned to the Ir-IV center and another (at more negative potential) is attributed to electron addition and removal from the tungsten-oxo framework that comprises the structure of each anion. These iridium-substituted heteropolyanions are shown to be excellent catalysts for the electroreduction of nitrite ion. The unsubstituted parent anions show no catalytic activity. The catalytic mechanism involves the rapid formation of a nitrosyl complex with the Ir-III form of the catalyst, which seems to depend on the electron configuration of Ir-III and the inorganic structure of the iridium-substituted heteropolytungstates. The pH dependence of the rate constant for the catalytic reaction shows that nitrous acid is the reactive form of the nitrite ion at pH < 3.0. The iridium-based heteropolytungstates are not degraded by repeated cycling between their oxidized and reduced states, which are attributed to their capacity to store and deliver multiple electrons to a bound substrate. A remarkable feature of the heteropolytungstates is that changes in the identity of the central heteroatom will produce systematic changes in the redox potentials of both the noble-metal center and the tungsten-oxo framework. Exploitation of this feature can provide decisive information in establishing the mechanism of electrocatalytic processes.