Cu-modified SAPO-34 molecular sieves with different Cu loading amounts were prepared by ion exchange and impregnation methods. Both series of catalysts were hydrothermally treated at 750 degrees C in 10 vol% H2O/air for 24 h. The catalysts were characterized by inductively coupled plasma optical emission spectrometry (ICP-OES), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), H-2 temperature-programmed reduction (H-2-TPR), NH3 temperature-programmed desorption (NH3-TPD) and diffused,reflectance infrared flourier transform spectroscopy (DRIFTS). All the fresh catalysts show high activity toward the selective catalytic reduction of NOx with NH3 (NH3-SCR) at low temperatures regardless of preparation method. Generally, the hydrothermal aging shifts the operation window of the catalysts towards high temperatures. The improvement in high-temperatures activity is attributed to the migration of surface CuO clusters into the ion-exchanged sites as isolated Cu2+ during ageing, which suppresses the competitive oxidation of NH3. The loss of low-temperatures activity is also related to the decrease in the amount of active surface CuO clusters, inhibiting the oxidation of NO to NO2 and fast SCR reaction. Interestingly, the amount of Lewis acid sites increase linearly with the increasing isolated Cu2+, while the Bronsted acidity is weakened. (C) 2016 Elsevier B.V. All rights reserved.