The mass transport mechanism of polypyrrole (PPy) and poly(N-methylpyrrole) (PMPy) films in an acetonitrile (AN) solution has been investigated with the cyclic electrochemical quartz crystal microbalance (EQCM) technique. Cations as well as anions take part in ion transport during the redox reaction of PPy films, and the break-in process occurs in the first negative scan. On the other hand, anion transport is dominant during the redox reaction of PMPy films, and no break-in process is observed. Solvent transport takes place in the same direction as cation transport in the case of PPy films, whereas it occurs in the opposite direction to anion transport in the case of PMPy films.