A statistical analysis of the infrared data obtained upon absorption of NO on a large number of Al-free Fe-silicalite and Fe-ZSM-5 samples highlighted the influence of Al on the formation and stabilization of extraframework iron species and shed more light on the superior activity of Fe-ZSM-5-based catalysts. It was concluded that Al favors the dispersion of extraframework iron species and that isolated Fell species with one or two Al atoms in the immediate vicinity are the active sites in partial oxidation reactions. The structure of the a-oxygen species adsorbed on iron centers is also briefly discussed.