A mono-stage device for generating gaseous sulphuric anhydride from melted sulphur, of the kind having two chambers, one inside the other and preferably concentric. The internal chamber contains spherical refractory material and has a duct feeding hot air into the bottom of the chamber. It opens at its upper end into the external chamber. Molten sulphur is fed into the upper end of the inner chamber and is burned to form SO.sub.2 which flows down in the external chamber containing catalytic material to form SO.sub.3 which is extracted through an outlet near the bottom of the external chamber.