Abstract: This invention relates to an apparatus and a method for the on-site generation of gas of relatively small quantities of a halogen gas, preferably chlorine gas. The apparatus has at least one electrolytic cell having an anolyte section and a catholyte section. At least one of these sections is connected, by fluid conduits, to a fluid heater which heats an electrolyte solution prior to its ingress into the section. Heating also facilitates circulation of the electrolyte solution through the apparatus by means of a thermosyphon effect. The heating element is, in turn, connected to an electrolyte replenishment means. The apparatus includes at least one gas separator which separates gas produced in the electrolytic cell from electrolyte solution. The apparatus lacks a reservoir for the storage of electrolyte solution.