A process for selective catalytic reduction of NOx in exhaust gases at temperatures of 200-600.degree. C. A spray of droplets containing a reductant is injected into a duct through an atomizing nozzle and into an exhaust gas. The exhaust gas, loaded with the spray droplets, is passed through the spaces between the plates of a series of stacks of parallel plates. The plates of each stack in the series are positioned at an angle relative to the walls of the gas duct and any adjacent stack, forcing the exhaust gas to pass through the stacks in a zig-zag flow pattern. After passing through the last stack, the exhaust gas then passes through a layer of monolithic catalyst for selective reduction of NOx by NH.sub.3 with channels parallel to the walls of the duct. Substantially all the spray droplets impinge on the walls of the plates, and the diameter of the droplets are larger than the critical diameter at which they are caused to touch the walls each time the direction of gas-flow is deflected upon passing through the stacks.