The effects of high temperature steam treatment on the activity and selectivity of CuZSM-5 catalysts for the selective catalytic reduction of NO with propene have been examined. Loss of activity and selectivity on steam treatment is due to framework dealumination of the zeolite, which causes migration of Cu out of the zeolite pores to the external surface. Pre-exchange of the zeolite with La3+ cations inhibits the dealumination, preventing migration of Cu to the surface and allowing the catalyst to retain high activity and N-2 selectivity after steam treatment. The deactivation of the catalysts was monitored using X-ray powder diffraction, magic angle spinning Al-27 and Si-29 nuclear magnetic resonance, X-ray photoelectron spectroscopy and ion scattering spectroscopy.