Electrochemical quartzcrystal microbalance (EQCM) studied revealed that the electrochemical properties of polypyrrole (PPY) in aqueous solutions are greatly dependent n the solution pH. A PPY film immersed in an aqueous KCl solution shows redox processed which involve not prevails as has been widely recognized. It has been found form comparative studies using poly (N=methylpyrrole) that protonation and deprotonation at nitrogen atoms of PPY play a key role in the observed pH-dependent electrochemicl behavior. Deprotonated PPY allows transient incorporation of hydrated cations upon reduction in weak alkaline aqueous solution. followed by alsow ejection of the incorporated cations when conventional electrolytes such as KCl and NaClO4 are used, but not in NH4Cl solution. Discussion on the catin transport in PPY is made in termas of deprotonation and protonation of nitrogen atoms of pyrrole subunits in PPY.