Iron- and iron + aluminum-substituted ferrierites (Fe-FER and Fe+Al-FER) have been characterized and their properties compared. Fe-FER exhibits excellent catalytic performance in mild oxidation processes, whereas Fe+Al-FER is hardly active. To specify the source of catalytic activity, in situ Mossbauer studies were performed to distinguish the various iron-containing species. Upon treatment in hydrogen performed on Fe-FER, the temporary presence of the Fe2+ state was evidenced in tetrahedral framework sites, and removal of iron did not occur in significant amounts from these lattice positions. In contrast, similar treatment on Al+Fe-FER results in permanent Fe3+ to Fe2+ reduction and in removal of iron in considerable amounts from the framework sites. From comparison of the behavior of samples and additional analysis of data, it is suggested that dinuclear Fe-lattice-O-Fe-extralattice centers are present in Fe-FER and that they have a primary role in mild oxidations.