Effects of cerium(III) ions are reported in this paper on the activity and hydrothermal stability of Cu-ZSM-5 catalysts for nitric oxide decomposition in water vapor-containing gas streams, Wet conditions drastically decrease the NO decomposition activity of unpromoted Cu-ZSM-5 catalysts at 500 degrees C. Activity is partially recovered upon removal of water vapor. Copper ion species migration leading to CuO formation inside the zeolite channels has been identified by STEM/EDX, XRD, and XPS as the major pathway to deactivation, The cerium modified Cu-ZSM-5 displays higher wet-gas activity for NO decomposition and recovers a higher fraction of its initial dry-gas activity than the Cu-ZSM-5 catalyst after removal of water vapor, The presence of cerium suppresses CuO particle formation and dealumination, providing higher hydrothermal stability for the zeolite structure and higher copper dispersion, all of which contribute to better catalytic performance of the Ce,Cu-ZSM-5 materials.