Abstract: The selective catalytic reduction of nitrogen oxides to nitrogen is effected by reacting nitric oxide, nitrogen dioxide or a mixture thereof with a reducing agent consisting of an aliphatic carboxylic acid having 1 to 5 carbon atoms at a temperature ranging from about 250.degree. to, about 600.degree. C., in the presence of a catalyst comprising a metal oxide selected from the group consisting of vanadium oxide, copper oxide, nickel oxide and iron oxide, the catalyst being supported on a porous carrier. The process of the invention enables one to substantially completely reduce NO.sub.x to harmless N.sub.2 in an efficient, environmentally friendly and cost-effective manner.