Abstract: The invention relates to the purification of gaseous effluents containing ides of nitrogen by reduction of said oxides in the presence of a catalyst at raised temperatures.The catalysts used in this process contain iron as a major constituent, and may contain chromium or vanadium: these constitute 96 to 99.5% of the weight of the catalyst. Also present as promoters, in a total amount of 4.0 to 0.5% by weight of the catalyst, are one or more rare earth metal oxides and a trace quantity of a platinum metal. The catalyst is prepared by a method which ensures a specific surface of 20-100 m.sup.2 /g and pores having a maximum dimension between 100 and 5000 A.Such a catalyst is particularly suitable for removing residual oxides of nitrogen from the gaseous effluent obtained in the production of nitric acid by the oxidation of ammonia.