A process is described for treating nitrogen-oxide containing gases at temperature in the range of 300-560.degree. C. in the presence of a cerium containing zeolite catalyst and an excess of an NH-containing compound such as ammonia or an aqueous urea solution as a reducing agent. Excess reducing agent is converted to nitrogen gas during the process. Selective catalytic reduction using ammonia water or an aqueous urea solution as a reducing agent is also described for converting nitrogen oxides in diesel exhaust gas and preventing oxidation of sulfur dioxide.