The mechanism of transitions from stationary to oscillatory currents has been studied for H2O2 reduction at Pt electrodes in acidic solutions. The transitions were observed in current versus time curves after the electrode potential was stepped from the rest potential to a potential where an oscillation appeared. The transitions could be classified into two types: a direct transition from a stationary to a final oscillatory current only, with slight modification in the amplitude and period of the oscillation at its initial stage; and an indirect transition via intermediary small-amplitude and short-period oscillations with complex waveforms. The direct transition was observed when the stepped potential was much more negative than the positive end of the oscillatory region in a phase diagram, whereas the indirect transition was observed when the stepped potential was close to the positive end. The appearance of the intermediary oscillatory currents with complex waveforms in the indirect transitions was mainly due to occurrence of small local oscillations at parts of the electrode surface and sudden extensions in the oscillatory area, induced by local deviations and fluctuations of the true electrode potential (or Helmholtz-layer potential), respectively. The main features of the intermediary oscillatory currents were reproduced by mathematical and experimental simulation.