One way to immobilize heteropolyanions on an electrode is to entrap them into a film of conducting polymer. In the present work, a polypyrrole film doped with a Keggin-type heteropolyanion, SiMo12O404-, obtained by electrochemical way was studied. We focus on the charge compensation process, which occurs when the heteropolyanions entrapped into the polymer are reduced or re-oxidized. First, electrochemical quartz crystal microbalance analyses were performed, then, to determine if solvent motion takes place, ac-electrogravimetry measurements were carried out. In fact, in concentrated acidic solution, only protons are involved in the charge compensation. In low acidic solution (pH 3) having a high potassium concentration (0.5 M) both proton and potassium ions are involved whereas at pH 5.2 only potassium motion takes place. These findings are in good agreement with the acido-basic properties Of SiMo12O406-. In all cases, no solvent motion was pointed out. (c) 2007 Elsevier Ltd. All rights reserved.