In this electrolysis apparatus for the preparation of chlorine from aqueous alkali halide solutions, which comprises at least one electrolytic cell, the anode and gas diffusion cathode are disposed horizontally and separated from each other by a partition in a housing consisting of two half shells and are connected mechanically and in an electrically conducting manner by means of their edges to the respective half shells. In addition, the cathode is connected in a liquid-tight manner to its half shell and is supported by a current supply and distribution device. The partition is clamped between a circumferential seal between the edges of the half shell and a frame and is supported on a spacer which rests on the cathode and is held by the frame. The frame contains an inlet and outlet for the catholyte.