The hydrothermal stability of Fe-BEA as a selective catalytic reduction (SCR) catalyst was experimentally studied. Cordierite supported Fe-BEA samples were hydrothermally treated at 600 and 700 degrees C for 3-100 h to capture the effect of aging time and temperature. Before and after aging the samples were characterized with BET, XPS, XRD, and NH3-TPD. The catalytic performance of the samples with respect to NO oxidation, NH3 oxidation, and NO reduction (NH3-SCR) was studied by flow reactor experiments to correlate changes of the catalytic performance with structural changes of the zeolite and the iron phases. The NH3-SCR experiments did not show any significant decrease in activity after a short time of aging (3 h at 700 degrees C) even though the ammonia storage capacity decreased by 40% and the oxidation state of iron slightly increased. A longer time of aging resulted in decreased activity for NO reduction during low temperatures (150-300 degrees C), while at higher temperatures (400-500 degrees C) the activity remained high. The results indicate that the NO reduction is more sensitive at low temperatures to changes in the oxidation state of iron caused by hydrothermal aging than at higher temperatures. Furthermore, a maximum in activity for NO oxidation and increased oxidation state of iron (Fe3+) indicate Fe2O3 particle growth.