A catalyst is described capable of inducing reduction in nitrogen oxide pollutants of exhaust gases from internal combustion engines and industrial plants. The catalyst composition is formed from a mixture of catalytically active aluminum oxides and a mixture of at least two metal oxides, one which is chromium oxide and the other is an oxide of an element of the First Transition Series, impregnated with about 1% by weight of an impregnant comprising a catalytically active noble metal compound. An optional constituent of the catalyst compositions includes 0.5-10% by weight of beryllium, magnesium, calcium, strontium or barium in the mixed oxide composition.