Abstract: A double cathode composed of a Ni screen cathode and a porous gas diffusion oxygen cathode with a porous diaphragm in between was constructed for the purpose of measuring the loss of current efficiency in an electrolytic cell for chlorate production. With this device the cathodic side reactions, i. e. reduction of ClO~-, HClO, etc., have been investigated, revealing the dependence of the loss of efficiency on the concentration of NaClO_3, the pH of the electrolytic solution and the presence of Na_2Cr_2O_7. The suggested method can also be applied to studying the side reactions and the loss of efficiency for the alkalichlorine cell and the electrolytic production of BrO_3~-, IO_3~-, ClO_4~-, etc.