A gas stream containing nitrous oxide and ammonia is contacted with a catalyst composition containing a zeolite. N.sub.2O is reduced to N.sub.2 and H.sub.2O at low temperatures in a highly efficient manner. Ammonia-mediated reduction of nitrous oxide can be effectuated from gas streams having N.sub.2O concentrations as low as 1%. The gas stream may also contact a catalytic composition selective for the reduction of NO.sub.x. In this way, N.sub.2O and N.sub.x treatment may be effectuated in a single process stream.