The present invention relates to a catalyst composition for selective catalytic reduction of NOx by ammonia or compounds, such as urea, generating ammonia under exhaust gas conditions, the composition comprising a) a copper ion exchanged zeolite particles having a Si/ Al2 molar ratio (SAR) of 15 or less and a copper content sufficiently high to perform the catalytic reduction, b) a nanocrystalline aluminium compound in an amount sufficient for stabilizing the zeolite, and c) a zirconium compound in an amount sufficient to improve hydrothermal durability of the catalyst composition.