Abstract: A combustion gas is cooled a first time to about 110.degree. C. in a first cooling chamber (3), up to 90% of the ashes is removed in a first precipitator (9) then the gas is cooled again in a second cooling chamber (10), to about 80.degree. C.; in a reaction chamber (13), by energization by means of electrons the sulphur dioxide and nitrogen oxides are converted into sulphuric acid and nitric acid and, with the addition of dry ammonia (15) the two said acids are converted to salts of ammonia, the latter are retained in a second precipitator (21) and sent to a chimney; characteristics of the method and the plant are that, from the instant of the injection of the ammonia into the reaction chamber (13), the temperature of the gas is maintained at above 80.degree. C.