The influence of anions on the electrodeposition, properties and overoxidation of poly(o-methoxyaniline), (POMA), is investigated by means of an electrochemical quartz crystal microbalance (EQCM) in the presence of HCl and HClO4 acids. The effect of anions during polymerisation is explained in terms of the interactions of anions with neutral or oxidised monomer and trapping of hydrated anions in the polymer films. The exchange of anions and the solvent molecules is studied for the doping/undoping process of POMA. The degradation of the polymer films is considered taking into account the presence of water molecules in the polymer films. Scanning electron microscopy (SEM) was used to detect the changes in polymer morphology caused by degradation.