The electropolymerization of o-phenylenediamine on gold and the redox behaviour of poly(o-phenylenediamine) films were studied in acid media using the electrochemical quartz crystal microbalance technique. It was established that the film formation is not an autocatalytic process and takes place mainly during the negative-going potential scan. The results indicate that the films are swollen by solvent molecules and ions in both reduced and oxidized states, and the extent of swelling increases with film thickness. At least two electron transfer steps should be considered in the redox mechanism. In the first oxidation step anion insertion and/or proton expulsion occur, while in the second oxidation step anions and/or protons are expelled. The magnitude of the mass changes accompanying the redox transformations depends on the pH of the bulk solution, which indicates that protonation-deprotonation processes play an important role in the overall mechanism. Dimerization of sites of different oxidation states may also occur. A counter-flux of water molecules can be assumed during the second oxidation step, i.e. the fully oxidized films are more hydrated.