[en] The production mechanism of liquid-phase H₂O₂ in dc driven plasma in O₂ and Ar with a water electrode was investigated. When a water anode was used, the concentration of H₂O₂ increased linearly with the treatment time. The production rate was proportional to the discharge current, and there was no dependence on the gap distance. On the other hand, the production rate was much smaller with a water anode. We concluded that the production of gas-phase H₂O₂ in the cathode sheath just above a water cathode and diffusion of this H₂O₂ into the water constitute the key mechanism in the production of liquid-phase H₂O₂. (paper)