Electrochemical polymerization of aniline onto a gold electrode surface in an aqueous electrolyte solution containing a small amount of polyelectrolyte, i.e., sodium poly(styrenesulfonate) (Na-PSS), and the characterization of the resulting polyaniline (PAn)/poly(styrenesulfonate) (PSS) composite have been studied using electrochemical quartz crystal microbalance and other electrochemical experiments. The entrapment of the polyelectrolyte affected the electrochemical properties, morphology, and structure of the polymer film as evidenced by the results of cyclic voltammetry and scanning electron microscopy experiments but did not affect the growth rate, as the increases in charge measured during the polymerization process are identical regardless of whether the polyelectrolyte is present or not. Surprisingly, the PAn/PSS composite thus formed had dominantly anion-exchanging characteristics, which is not in agreement with what has been generally believed for this type of composite polymer. The ion-exchanging properties were found to be pH dependent.