The method comprises passing an N.sub.2O- and NO.sub.x-containing gas over a sequence of two catalyst beds, adding reducing agents for NO.sub.x and for N.sub.2O between the catalyst beds in such an amount that not only NO.sub.x but also a predetermined proportion of N.sub.2O is reduced. The reaction conditions are set so that the N.sub.2O content of the gas is reduced by not more than 95%, based on the N.sub.2O content at the entrance of the first catalyst bed, by decomposition to nitrogen and oxygen in the first catalyst bed and that not only chemical reduction of NO.sub.x but also chemical reduction of N.sub.2O occurs in the second catalyst bed so that the N.sub.2O content of the gas is reduced by at least 50%, based on the N.sub.2O content at the entrance of the second catalyst bed.