The invention relates to a novel catalyst having excellent activity, selectivity and stability for reducing nitric oxide to gas nitrogen, with hydrogen being used as a reducing agent, in the low temperature range 100 200.degree. C. and in the presence of an excess of oxygen (e.g. 5% vol), H.sub.2O (5% vol) and/or SO.sub.2 (20 ppm) in the supply. The inventive catalyst consists of platinum crystals which are in contact with the phases of a mixed MgO and CeO.sub.2 medium or in the form of platinum supported on the mixed MgO--CeO.sub.2 medium which has already been sulphated in a selective manner. The Pt/MgO--CeO.sub.2 catalyst can be used to obtain NO conversion levels which are greater than 40% and nitrogen selectivity values of greater than 80% under NOx oxidation conditions in the 100 400.degree. C. temperature range and for a surface contact time of 0.045 s. In particular, full NO conversion is obtained as well as N.sub.2 selectivity levels of 83% at 150.degree. C. with a reaction mixture of 0.25% NO/1% H.sub.2/5% O.sub.2/5% H.sub.2O/He.