Polypyrrole films containing either the perchlorate or p-toluenesulfonate ion were prepared from an aqueous solution by applying an anodic potential in the range 0.6-2.0 V (relative to a saturated calomel electrode). The effects of applied potential on electropolymerization, morphology, chemical structure, electrochemical properties and tensile modulus have been investigated. The chronoamperometric response of pyrrole electropolymerization depends on the applied potential and not on the counterion. The electrochemical properties and structure of polypyrrole depend on the polymerization potential as well as the counterion. An empirical expression relating peak current to the scan rate and charge passed during polymerization is proposed. This relationship provides some insight into the nature of the polypyrrole switching reaction. For polypyrrole containing perchlorate, the concentration of dicationic species or dipolarons decreases as the polymerization potential increases from 0.6 to 1.0 V. The large concentration of dicationic species in polypyrrole prepared at 0.6 and 0.75 V is responsible for the second reduction peak observed in the cyclic voltammograms.