This catalyst system simultaneously removes ammonia and enhances net NO.sub.x conversion by placing an NH.sub.3--SCR catalyst formulation downstream of a lean NO.sub.x trap. By doing so, the NH.sub.3--SCR catalyst adsorbs the ammonia from the upstream lean NO.sub.x trap generated during the rich pulses. The stored ammonia then reacts with the NO.sub.x emitted from the upstream lean NO.sub.x trap--enhancing the net NO.sub.x conversion rate significantly, while depleting the stored ammonia. By combining the lean NO.sub.x trap with the NH.sub.3--SCR catalyst, the system allows for the reduction or elimination of NH.sub.3 and NO.sub.x slip, reduction in NO.sub.x spikes and thus an improved net NO.sub.x conversion during lean and rich operation.