The promotion of In-ZSM-5 catalysts for the selective catalytic reduction of NO by methane by CeO2 has been investigated by a combination of physicochemical techniques (XPS, EXAFS, TPR, XRD, FTIR) and reaction studies, in particular under variation of the activation conditions. CeO2 is a highly effective promoter for In-ZSM-5 in dry feed (e.g., X(NO) > 70% at 100,000 h(-1) between 723 and 773 K with 1000 ppm NO, 1000 ppm CH4, and 2% O-2) whereas in moist feed, the promotional effect is pronounced only at high temperatures. The role of CeO2 in the reaction mechanism is to provide a sufficient NO2 supply on the basis of its high NO oxidation activity. Under catalytic conditions, CeO2 undergoes interaction with the zeolite component, which stabilizes part of the Cc ions in the +3 state in an oxidative atmosphere while the majority of Cc ions remain in the CeO2 lattice. This effect does not appear to be constitutive for the catalytic function of the CeO2 promoter. In-ZSM-5 catalysts with different active site structures exhibit different responses to promotion by ceria. Intrazeolite indium oxo species, which are often considered the most important active sites in In-ZSM-5 SCR catalysts, are not among the most favorable cases upon promotion with CeO2. Intense promotional effects are found with sites that catalyze unselective methane oxidation in the absence of CeO2 (intrazeolite InOxCly species, extrazeolite In2O3 aggregates, probably active at the borderline with the zeolite). Ternary mechanical mixtures of CeO2, In(OH)(3), and NH4-ZSM-5 also exhibit high SCR activities provided that activation is performed in a nonoxidizing atmosphere. TPR, FTIR, XRD data, and results of experiments with separate activation of mixture components suggest that the favorable performance of these mixtures arises from extrazeolite indium sites, which are probably at the borderline between In2O3 aggregates and external zeolite surface and are possibly engaged in In-O-Si and In-O-Al bridges. (C) 2004 Elsevier Inc. All rights reserved.