A radial flow gas phase reactor for the selective catalytic reduction of nitrogen oxide in a gas stream includes a shell enclosing an interior space in which is located at least one catalyst bed containing a catalyst for the selective conversion of NO.sub.x. A deflector directs the flow of gas radially through the bed. An injector upstream of the catalyst introduces a reducing agent such as ammonia into the inlet gas stream. The catalyst bed can include particulate, monolith, or microengineered catalyst.