In addition, the present invention relates to an apparatus for the continuous catalytic complete or partial oxidation of a starting gas containing sulphur dioxide and oxygen, characterized by at least one tube contact apparatus which is an upright heat exchanger composed of at least one double-walled tube whose catalyst-filled inner tube forms a reaction tube, with heat being transferred in cocurrent around the reaction tube and an absorber for separating off SO3 being installed downstream of the tube contact apparatus.
