A chlorinator for electrolytically producing chlorine from brine comprises an electrolytic cell having a vertical cation permeable diaphragm separating the anode and cathode compartments, a source of brine feeding concentrated brine into the anode compartment at regular intervals, and a mixing device in which chlorine gas produced by the electrolytic cell is mixed with a stream of water to be chlorinated. The electrolytic cell is provided with means for intermittently transferring small amounts of the anolyte from the anode compartment to the cathode compartment and for preventing the catholyte from coming into the anode compartment.