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 platinized platinum electrode. In the presence of Cu2+-ions an increase of the frequency response and a decrease of the period time were observed. At a given concentration range of Cu2+-ions the oxidation process also exhibited bistability : high-potential states and low-potential states. The observation of mixed oscillations indicates the existence of at least two Feedback mechanisms. As it is generally assumed, the principal mechanism involves the nonlinear and autocatalytic oxidation reaction between the strongly bound, inhibiting species formed by the chemisorption of formic acid and the adsorbed H2O molecules or OH radicals. Since the appearance of the mixed-mode oscillation is connected with the presence of CU2+. ions, another feedback mechanism should be considered in which Cu-adatoms and Cu2+-ions participate. The decrease of the period time upon addition of CU2+-ions may be explained in terms of the virtual increase of the anodic current at low potentials as a consequence of the electron consuming nature of this process.