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Selective catalytic reduction (SCR) of NOx by NH3 is under intensive development as a technology to enable diesel engines to meet stringent NOx emission regulations. Cu/zeolite SCR catalysts are leading candidates because of their ability to catalyze NOx reduction at the low temperatures encountered on many diesel vehicles. However, both engine evaluation and laboratory studies indicated that commonly available Cu/zeolite SCR catalysts did not have sufficient thermal stability to maintain performance during the full useful life of a vehicle (with steady-state NOx conversion decreasing ~ 10% over 64 hours of hydrothermal aging at 670 °C). Characterization of aged Cu/zeolite catalysts revealed that the loss of zeolite acidity was the main deactivation mechanism; while the zeolite support maintained its framework structure and surface area after aging. Improvement of the hydrothermal stability of the acid sites resulted in a new generation of SCR catalysts.