A bipolar cell for a reactor for treatment of electrolyte such as waste water and effluent or for electrosynthesis comprises end electrodes and at least one bipolar electrode therebetween. The or each bipolar electrode comprises a diamond sheet. The cell includes a porous support structure, for example in the form of spacers, a lattice of plastic rods, or a woven mesh, between each end electrode and the adjacent diamond sheet, there being porous support structure between the or each pair of adjacent diamond sheets, the support structures acting to contact or support the or each diamond sheet.