In this paper, the electrosynthesis of polypyrrole (PPy) films doped with a Keggin-type heteropolyanion, SiMo12O404−, is reported. The in situ electrochemical growth of the hybrid film was conducted using an electrochemical quartz crystal microbalance (EQCM) that allowed us to evaluate the doping level of the polymers, which were synthesized in galvanostatic mode. By combining EQCM with chemical analyses, such as X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy, it was evidenced that the doping level of the polymer depends on the current imposed during the PPy film synthesis. Copyright © 2006 John Wiley & Sons, Ltd.