Abstract: The object of the invention is to provide a flow-through, electric, double-layer capacitor providing for a high, steady rate of removal of ionic substances and amenable to commercial scale application and a method of treating fluids using the flow-through electric double-layer capacitor. The invention is a planar, flow-through, electric, double-layer capacitor comprising a separator (1) consisting in an electrically-insulating, porous, flow-through sheet, activated carbon layers (2, 2), each comprising a high specific surface area activated carbon as a main component, collectors (3, 3) disposed externally of the active carbon layers (2, 2), and retaining plates (4, 4) disposed externally of the collectors (3, 3).