Producing chlorine dioxide, comprises: reacting chlorite with acid in a reactor (3) in aqueous solution and removing the produced gaseous chlorine dioxide using a vacuum pump (10) from the reactor, where the reactor is subdivided into at least two reactor chambers by at least one partition wall comprising a porous material for the reaction of chlorite with acid. The chlorite and acid are supplied as starting materials into the first reactor chamber (5) respectively via separate lines using the flow limiting means and the vacuum of the vacuum pump. Producing chlorine dioxide, comprises: reacting chlorite with acid in a reactor (3) in aqueous solution and removing the produced gaseous chlorine dioxide using a vacuum pump (10) from the reactor, where the reactor is subdivided into at least two reactor chambers by at least one partition wall comprising a porous material for the reaction of chlorite with acid. The chlorite and acid are supplied as starting materials into the first reactor chamber (5) respectively via separate lines using the flow limiting means and the vacuum of the vacuum pump, where gas is supplied into the second reactor chamber (6). The formed gaseous chlorine dioxide in a mixture with the supplied gas is removed from the first reactor chamber and is transferred to fresh water. An independent claim is also included for a device for producing chlorine dioxide, comprising a reactor for reacting chlorite with acid and an apparatus for removing the produced gaseous chlorine dioxide from the reactor, where the apparatus comprises a vacuum pump and a gas feed, and the first reactor chamber for removal gaseous chlorine dioxide is connected to the vacuum pump.
