Electrically conducting polypyrrole (PPy) doped with heteropolyanions (HPA) [PMo12O40](3-) was synthesized by electrochemical polymerization. The electrical conductivity could be as high as 10(0) S cm(-1). Cyclic voltammograms showed reversible redox peaks from the polymer itself and from the immobilized HPA. Electrochemical quartz crystal microbalance measurement indicated that the anions were retained in the polymer matrix without being replaced after repeated potential cycles in electrolyte solutions containing no HPA. We also examined the electrocatalytic effects of the electrodes modified by this PPy + HPA film on the reduction of oxygen and chlorate.