Redox electrochemistry of tetra-iron substituted sandwich-type heteropolyoxometalates with the formula of Na-12[Fe-4(III)(H2O)(2)((2),W15O56)(2)]. 58H(2)O has been studied in acid buffer solutions using polarography and cyclic voltammetry. This new transition metal-substituted heteropolyoxometalate is one of the trivacant Dawson derivatives. It exhibited a one-electron couple associated with the Fe(III) center followed by two steps of four- and two-electron redox waves attributed to redox processes of the tungsten-oxo framework. The new catalyst showed an electrocatalytic effect on the reduction of H2O2 and NO2- The tetra-iron-based heteropolytungstate was not degraded by repeated cycling between its oxidized and reduced states, which was attributed to its capacity to store and deliver multiple electrons to a bound substrate.