The invention relates to a process for the production of chlorine dioxide in an apparatus comprising a reaction vessel holding a reaction medium and a circulation conduit leading through a heater for the reaction medium, said process comprising the steps of continuously: a) maintaining the reaction medium at subatmospheric pressure and a temperature sufficient for evaporating water; b) circulating reaction medium through the circulation conduit and the heater; c) reacting alkali metal chlorate, sulfuric acid and hydrogen peroxide in the reaction medium to form chlorine dioxide, oxygen and alkali metal sulfate; d) feeding alkali metal chlorate, sulfuric acid and hydrogen peroxide to the reaction medium, wherein the sulfuric acid is fed through an inlet into the circulation conduit and the hydrogen peroxide is fed through an inlet directly into the reaction vessel or through an inlet into the circulation conduit down-streams the inlet for sulfuric acid at a distance in the flow direction therefrom of at least about the inner diameter of the circulation conduit; and, e) withdrawing a gas comprising chlorine dioxide, oxygen and evaporated water. The invention further relates to an apparatus for the production of chlorine dioxide.