An in situ combination of surface plasmon resonance (SPR) spectroscopy and quartz crystal microbalance (QCM) is used to study the electropolymerization and the doping/dedoping behavior of thin poly(pyrrole) (ppy) films in aqueous solutions. A mixed anion and cation exchange behavior is observed. The mass as determined with the QCM continuously increases during redox cycling. The combined QCM/SPR data reveal that this is caused by a relatively slow process occurring when ppy is in the oxidized (polaronic) state. This behavior is interpreted as an accumulation of neutral salt and solvent in the film.