Strain changes of polypyrrole (PPy) during electropolymerization and redox (reduction and oxidation! switching in perchlorate aqueous solutions were investigated using strain gages. It was found that the magnitude of strain changes during redox switches of PPy depends on the counter cations used during electropolymerization. For PPy films of the same thicknesses, PPy synthesized in LiClO4 demonstrated the largest strain changes during redox switching and, as the cation size increased, the strain changes became smaller. Because, regardless of the identity of counter cations, PPy showed similar strain behaviors in perchlorate solutions during redox switching, it is believed that the cations are involved in the electropolymerization process and likely to affect the formation of polymer chain structures. (C) 2003 The Electrochemical Society.