Thin films of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with heteropolyacids have been grown electrochemically and by a vapour transport method onto carbon paper supports, both methods employing 12-molybdophosphoric acid and 10-molybdo-2-vanadophosphoric acid as dopants. Coatings deposited potentiodynamically at 100mVs−1 from mixed 3,4-ethylenedioxythiophene (EDOT) and 12-molybdophosphoric acid or 10-molybdo-2-vanadophosphoric acid gave electrodes stable beyond 1000 voltammetric cycles in aqueous sulfuric acid; vapour grown electrode coatings degraded during cyclic voltammetry.