An electrolytic cell (1) is arranged to generate hydrogen peroxide and includes a reservoir (12) for storing hydrogen peroxide generated by the cell. A controller (19) is arranged to activate the cell and introduce stored hydrogen peroxide from the reservoir to the cell. This permits the concentration of a store of hydrogen peroxide to be refreshed as required, by introducing the stored hydrogen peroxide as the electrolyte in the cell. This counteracts the natural decay of the stored hydrogen peroxide. The pH of the hydrogen peroxide is controlled so that it is below a predetermined value. It has been found that hydrogen peroxide having a low pH, close to neutral, decays much slower than hydrogen peroxide having a more strongly alkaline pH.
