Abstract Nitric oxide adsorption on iron-exchanged, silicon-substituted Y-zeolite, iron-exchanged mordenite, and iron-exchanged ZSM-5 has been studied by both infrared and Mössbauer spectroscopies. In addition, the catalytic activities of these zeolites for nitrous oxide decomposition have been determined. The results show that NO adsorption is qualitatively the same for all the zeolites. However, the framework differences and the locations of the exchangeable cations control the relative concentrations of the various nitrosyl species formed upon adsorption. Zeolites that have a large fraction of the iron in accessible, low-coordination sites form predominantly mononitrosyls. Those zeolites that have a large fraction of iron in inaccessible, high-coordination sites form predominantly dinitrosyls and form them slowly. Measurements of catalytic activity and NO adsorption capacities correlate well with the relative accessibilities of the cations to reactant molecules. This indicates that the active sites for reaction are associated with those sites which form mononitrosyl species with NO. Thus, in Y-zeolite differences in activities can be accounted for by the changes in cation siting brought about by changes in the framework aluminum content.