Abstract: An apparatus and method for measuring and controlling the NOx and ammonia slip content of a NOx-containing gas stream such as, for example, a combustion engine exhaust stream discharged from a Selective Catalytic Reduction (SCR) system. The apparatus includes an analyzer container which preferably has an ammonia slip catalyst element and a pair of automotive type NOx sensors positioned therein. One of the NOx sensors is positioned before and the other is positioned after the ammonia slip catalyst. The apparatus is heated by positioning the container in the gas stream and can draw a representative gas sample from any size conduit.