Abstract: In flue gases or other humid exhaust gases which are relatively cold and contain SO.sub.2, NO.sub.2 and other gaseous pollutants, the SO.sub.2 content is oxidized to SO.sub.3 and the NO content is reduced by a catalytic treatment, the SO.sub.3 content is condensed as sulfuric acid and other gaseous pollutants are removed by being scrubbed with aqueous liquids at low temperatures. In order to heat up the flue gas to the temperature required for the catalysis, the flue gas is heated up in a first heating-up stage by an indirect heat exchange with the catalytically treated gas before the catalytic treatment, the heated-up gas is heated up further in a second heating-up stage to the temperature required for the catalytic treatment, the catalytically treated gas which has been cooled in the first heating-up stage is cooled further in an indirect heat exchanger below the dew point temperature of sulfuric acid and is subsequently fed to an SO.sub.3 condenser.

Abstract: Flue gases or other contaminated humid exhaust gases which contain SO.sub.3 are cooled by an indirect heat exchange in a heat exchanger to a temperature below the dew point temperature of sulfuric acid and are cooled further to a predetermined temperature in an SO.sub.3 condenser by means of dilute sulfuric acid, which is sprayed to flow in a countercurrent to the gas, whereby dilute sulfuric acid having a predetermined concentration is condensed. The gas leaving the SO.sub.3 condenser is scrubbed and cooled to a temperature from 40.degree. to 55.degree. C. in a fine scrubber by a treatment with a sprayed aqueous liquor. The exit gas is reheated to a predetermined temperature in a reheater by a treatment with sprayed dilute sulfuric acid from the SO.sub.3 and after the condensation of SO.sub.3 is passed through a filter. Dilute sulfuric acid from the reheater is sprayed into the SO.sub.3 condenser.