Silicalite-supported indium catalysts (In/H-ZSM5) were characterized by time differential perturbed angular correlation (PAC) and temperature-programmed reduction (TPR). The presence of different indium species was correlated with activity and selectivity during the NO selective catalytic reduction (SCR) with CH4 in the presence of excess oxygen. The main species identified on the In/H-ZSM5 surface were In2O3 (indium sesquioxide crystals); In(+)Z(-) and (InO)(+)Z(-) (different indium species exchanged in the zeolitic matrix); and highly dispersed noncrystalline In oxide species not bonded to the zeolitic matrix. Catalysts that were impregnated and then calcined at 500 degrees C had low activity for the reaction under study, showing the presence of only In2O3 and noncrystalline In oxide species. Treatment at 750 degrees C in O-2 or at 500 degrees C in H-2 followed by reoxidation at the same temperature resulted in active catalysts showing an appreciable concentration of (InO)(+)Z(-) active species. The same active species were formed after indium ion exchange of NH4-ZSM5 was followed by calcination at 500 degrees C. The PAC technique proved to be a powerful tool for the identification and quantification of indium species present on the surface of an H-ZSM5 support.