Abstract: A reactor (1) is described in which a catalytic reaction, associated with a thermal effect, is performed on substances contained in a gas flow, in particular the recovery of sulphur from gases containing hydrogen sulphide. A housing (2) of the reactor (1) comprises a gas inlet (6) and a gas outlet (9). Inside the housing (2) is arranged a bed of catalytically active material, heat being supplied to or withdrawn from said material by means of a temperature regulating device (23) depending upon the nature of the reaction. The temperature regulating device (23) is formed by a plurality of tubes (24) bent in a U-shaped formation. In each case one side of all these U-tubes (24) leads into a first distribution chamber (17) which can be supplied with a heat carrier medium. The other side (24aa, 24ba) of the U-tubes (24) in each case leads into a second distribution chamber (18) from which the heat carrier medium can be discharged again.