A catalyst compositions for reducing nitrogen oxide comprising, as its chief ingredient, an intimate mixture ofA. titanium (Ti) as component A, withB. at least one metal selected from the group consisting of molybdenum (Mo), tungsten (W), iron (Fe), vanadium (V), nickel (Ni), cobalt (Co), copper (Cu), chromium (Cr), and uranium (U), as component B,in the form of their oxides, and a process for reducing nitrogen oxides to nitrogen, which comprises contacting a gaseous mixture containing nitrogen oxides and molecular oxygen and a reducing gas with aforesaid catalyst compositions at an elevated temperature.