The present invention is directed to an iron-promoted zeolite beta catalyst useful in the selective catalytic reduction of nitrogen oxides with ammonia in which the iron-promoted zeolite beta is treated so as to provide increased amounts of the iron promoter in the form of Fe(OH).The stabilized zeolite is formed by cation exchange of an iron salt into a zeolite beta which has a reduced sodium content such as by exchanging a sodium beta with ammonium or hydrogen cations. A zeolite beta having a reduced carbon content and a Si/Al ratio of no more than 10 also increases the Fe(OH) content of the iron-promoted catalyst. The iron-promoted catalyst which has the iron in the form of Fe(OH) is characterized by a peak at 3680.+-.5 cm.sup.-1 in the IR spectra.