The effects of high-temperature steam treatment on the activity and selectivity of CoZSM-5 catalysts for the selective catalytic reduction of NO with methane have been examined. Loss of activity and selectivity on steam treatment is due to framework dealumination of the zeolite, causing an irreversible change in the coordination state of the Co2+ ions. Pre exchange of the zeolite with La3+ cations inhibits the dealumination while stabilizing the catalysts.