In a production mode a process for preparing sulfuric acid may involve oxidizing sulfur to sulfur dioxide in a first oxidation stage, and catalytically oxidizing the sulfur dioxide to sulfur trioxide in a second oxidation stage. The sulfur trioxide may be absorbed in at least one absorption stage. In the production mode, process gases from a last of the at least one absorption stage with respect to a flow direction are discharged. In a standby mode of the process, at least one heating stage for heating the process gases is connected. The process gases exiting from the at least one absorption stage are conveyed to the heating stage, and the process gases are circulated via the heating stage, the second oxidation stage, and the absorption stage.