A practical-scale cell (0.8 m(2)) with a trickle-bed electrode was developed for on-site electrochemical production of hydrogen per-oxide by the cathodic reduction of oxygen. Terminal voltage and current efficiency were affected by the dispersions of liquid and oxygen gas in the trickle-bed cathode and by the current density. Current efficiency was improved by obtaining uniform dispersions of electrolyte and oxygen in the trickle-bed cathode. A current efficiency of 97.4% was obtained at a cell voltage of 2.1 V and a current density of 1500 A m(-2). To save power, the temperature of the electrolyte was not controlled, and exothermic reactions increased the temperature of the anolyte recovered at the outlet from the inlet temperature of 37-44 degrees C. The temperature of the catholyte rose simultaneously from an inlet temperature of 25-44 degrees C. The crystallization of sodium peroxide within the cathodes was prevented by controlling peroxide concentration, and the maximum concentration of hydrogen peroxide in a 5.0% caustic soda solution was 2.1%.