The conducting polymer, polypyrrole was successfully employed to surface immobilise the Krebs type polyoxometallate, K6NaH[Sb2W20Fe2O70(H2O)(6)]center dot 29H(2)O, through cyclic voltammetry. The resulting K6NaH[Sb2W20Fe2O70(H2O)6]center dot 29H(2)O doped polypyrrole films were found to exhibit the redox activity associated with both the Fe(III) and W-O redox centres within the POM. The former was found to be situated at potentials within the conducting part of the polymer whilst the latter redox process was in the insulating domain of the polypyrrole. The Fe(III/II) POM based redox process was found to be pH dependent. Upon redox switching of the polymer through this Fe(III/II) redox system, a process of cation insertion and expulsion into the polypyrrole matrix was observed with both the nature and concentration of the supporting electrolyte having a substantial effect upon the potential values at which this process occurred. This cation insertion-expulsion process was investigated through the application of the Electrochemical Quartz Crystal Microbalance (EQCM) technique. The result of which indicated that it was both the passage of alkali metal cations and protons from the background electrolyte into the polymer film which maintained electroneutrality within the POM polypyrrole films upon redox switching through the Fe(III/II) redox system. Finally the POM doped polypyrrole films exhibited a clear catalytic property towards the reduction of hydrogen peroxide (H2O2) with a sensitivity of 131.8 (+/- 3.5) mu A cm(-2)/mM with LOD of 16.6 mu M at neutral pH. (C) 2008 Elsevier Ltd. All rights reserved.