Electrochemical quartz crystal microbalance (EQCM) studies revealed that the electrochemical properties of polypyrrole (PPY) in aqueous solutions are greatly dependent on the solution pH. A PPY film immersed in an aqueous KCl solution shows redox processes which involve not only anion transport but also cation transport if the solution pH is greater than 3-4, whereas at lower pH values only anion transport prevails as has been widely recognized. It has been found from comparative studies using poly(N-methylpyrrole) that protonation and deprotonation at nitrogen atoms of PPY play a key role in the observed pH-dependent electrochemical behavior. Deprotonated PPY allows transient incorporation of hydrated cations upon reduction in weak alkaline aqueous solutions, followed by slow ejection of the incorporated cations when conventional electrolytes such as KCl and NaClO4 are used, but not in NH4Cl solution. Discussion on the cation transport in PPY is made in terms of deprotonation and protonation of nitrogen atoms of pyrrole subunits in PPY.