A novel electrochemical approach was developed for the kinetic study of the formation of heteropolyanions. The method (dual pulse amperometry, DPA) is based on the detections of currents due to the transfers of polyanions at the nitrobenzene-water interface. In this study, DPA was applied to the kinetic study of the formation of two Keggin anions, viz., [SiMo12O40](4-) and [GeMo12O40](4-). Prior to the kinetic study, cyclic voltammetric measurements were performed to confirm that the Keggin anion and its lacunary anion ([H3SiMo11O39](5-)) or ([H-3/GeMo11O39](5-)) coexist at equilibrium under certain conditions. In DPA, double voltage pulses of different amplitudes were alternately applied to the interface to follow the concentrations of both the Keggin and the lacunary anions. The concentration-time profiles for the polyanions could be elucidated by the two-step consecutive reactions mechanism. The lacunary anion was then found to be the intermediate of the Keggin anion.