The successful use of conducting polymers for actuators depends on the ability to control force and position precisely by the application of a potential. The reversible oxidation and reduction of the polymer backbone is accompanied either by the insertion/expulsion of anions, by the expulsion/insertion of cations, or by a more complicated mixture of the two cases. The identity of the mobile ion has been elucidated for three polypyrrole (PPy) based systems by using the Nernst equation to interpret the dependence of the peak potentials in voltammograms on electrolyte concentration. Using the LiCIO4,/propylene carbonate electrolyte, the CIO4- ion is the main mobile species, whereas when using LiCIO4/acetonitrile it is the Li+ ion. The switch in mechanism caused by the change of solvent, which does not formally enter into the oxidation/reduction equation, shows that the effect depends on finely balanced interactions between the ions, the polymer and the electrolyte solvent. When using a large, immobile anion (dodecyl benzene sulphonate) incorporated in PPy, the results in the NaCl/H2O electrolyte indicate mainly Na- ion motion, as expected. Simultaneous cyclic voltammetry and quartz crystal microbalance measurements indicate that the Na+ ions are accompanied by a large number (10-20) of H2O molecules. Force measurements show that the full variation in force occurs over a relatively small potential range.