The details of a potentiostatic preparation method for a composite of iron(III) hexacyanoferrate and poly(3,4-ethylenedioxythiophene) on a platinum electrode are reported. The heterogeneous electron transfer processes involving the composite and its stability are examined in aqueous and acetonitrile solutions. Cyclic voltammetrical studies show that in aqueous solution poly(3,4-ethylenedioxythiophene) is in its neutral state (non oxidised state) in the potential range where iron(III) hexacyanoferrate is electroactive. On the other hand, in acetonitrile solution, we show that iron(III) hexacyanoferrate is not electroactive. Thus, the faradaic response of the composite is due only to the poly(3,4-ethylenedioxythiophene) in this organic medium. This behaviour allows us to study separately the electrochemical behaviour of each part of the composite in the two solvents. A cooperative effect (synergy) is observed within the composite. This is reflected by the increase of the faradaic and the capacitive responses of the poly(3,4-ethylenedioxythiophene) in acetonitrile solution in the presence of iron(III) hexacyanoferrate. Moreover, the stability of the iron(III) hexacyanoferrate in the composite material is enhanced in aqueous solution.