The present invention is directed to a method for removal by oxidation of the excess ammonia (NH.sub.3) gas ("ammonia slip") resulting from flue gases that have been subjected to selective catalytic reduction (SCR) for reduction of nitrogen oxides (NOx) with ammonia. More specifically, the inventive method uses an ammonia oxidation catalyst consisting of a zeolite, one or more precious metals, and a base metal compound, to catalyze the oxidation of both ammonia and carbon monoxide (CO), while minimizing the formation of nitrogen oxides (NOx). The present invention is useful in treating flue and exhaust gases.