Abstract Simultaneous periodical changes of the potential and the electrochemical quartz crystal frequency response were detected in the course of galvanostatic oxidation of formic acid at a platinized platinum electrode and at a platinized platinum electrode partially covered with poly(aniline) film, respectively. The poly(aniline) layer on the metal surface acts as a pseudocapacitance, and the presence of this electrochemical capacitive element increases the period time of the oscillation. It provides an easy method to regulate the oscillation and also to gain a deeper understanding of the effect of the electrode capacitance on the electrochemical oscillation reactions. Simultaneously with the potential oscillation, periodical changes of colour of the electrode can be observed due to the electrochromic nature of the poly(aniline) redox reaction. The periodical mass changes detected by quartz crystal microbalance are enhanced due to the sorption/desorption of counterions accompanying the redox transformation of poly (aniline).