Abstract: An oxido-reduction process for nitrogen oxides contained in a gas stream also containing oxygen, in a countercurrent gas-liquid contact device. In this process,(a) for values of the ratio NO/NO.sub.2 greater than 1, in that section of the gas-liquid device where the NO.sub.x content in the gas stream is less than about 6000 vpm, the ratio K, which is the ratio of the liquid retention volume in m.sup.3 to the flow rate of the gas stream in Nm.sup.3 /hour, is increased so that the NO.sub.x are preferentially absorbed according to the reaction: NO+NO.sub.2 +H.sub.2 O.fwdarw.2HNO.sub.2 (liquid phase); and(b) oxidation is then induced, to reduce the value of the ratio NO/NO.sub.2 and to restore the conditions of stage (a).In practice, the operating point describes a curve A.sub.0 A.sub.2 B.sub.1 A.sub.3 B.sub.2 A.sub.4, as shown in FIG. 1.