Exchange of O-18(2) With O atoms in two groups of Fe containing ZSM-5 catalysts was studied. One class was prepared by sublimation of FeCl3 onto H-ZSM-5 with various Si/Al ratios. These materials are highly active and selective catalysts in the reduction of NO, with alkanes. A second group, further called Fe2O3/ZSM-5, was prepared by impregnation. Significant differences in the exchange behavior were found between both groups; the exchange rate is higher for Fe2O3/ZSM-5 than for Fe/ZSM-5, in accordance with the higher selectivity for alkane combustion of Fe2O3/ZSM-5 and its low selectivity for NOx reduction. The data suggest that negligible amounts of iron oxide particles exist in the Fe/ZSM-5 catalyst. For Fe/ZSM-5 catalysts the exchange is not limited to the oxygen aroma directly attached to iron; some zeolite oxygen is also exchanged, although this does not occur for the metal-free zeolite. Kinetic analysis shows that with Fe2O3/ZSM-5 the R-2 exchange mechanism prevails in which O-18(2) is exchanged in one step against two O-16 atoms. For the selective Fe/ZSM-5 catalyst R-1 exchange prevails at low temperature, i.e., O-18(2) molecules exchange only one O atom against an atom of the solid. The qualitative difference between both types of catalyst indicates that Fe/ZSM-5 catalysts owe their selectivity in NOx reduction to Fe ore-ions.