The catalytic decomposition of ammonia was studied over zeolite supported iron catalysts regarding the hot gas cleaning requirements of coal/biomass to liquid processes. Catalysts were prepared on different zeolite frameworks (HZY, HZ beta and HZSM5 with SiO2 to Al2O3 ratios of 5.2, 38 and 280) and tested. Brighter grains in the SEM images of Fe/HZSM5 catalyst were noticed to be the iron aggregates on the external surface. Any indicative brighter grains were not detected in the SEM images of Fe/HZ beta and Fe/HZY catalyst meaning that iron might be dispersed well and attached to the aluminum sites. The FeOx cluster size was considered to be dependent on the textural properties of the zeolites. The absence of alpha-iron (bcc) phase on the Fe/HZY catalyst upon reduction was interpreted as the strong interaction of iron with the aluminum sites of HZY. This interaction might provide the stabilization of FeO phase by preventing its further transformation to metallic iron. The reduction of stabilized FeO to metallic iron on Fe/HZY catalyst upon reaction seemed to involve a reaction between iron oxides and non-framework aluminum species like Al(OH)(2+), Al(OH) (2) (+) , AlO+, AlOOH etc. leading to iron aluminate (FeAl2O4)-like clusters. The highest activity over the Fe/HZ beta catalyst might be due to the better hydrothermal stability of zeolite H beta compared to zeolite HY regarding their aluminum contents. The role of the zeolite was probably to maintain good dispersion of active iron clusters without considering support effect on N-H bond breaking. Therefore, the best coordination environment for active iron species and improved iron cluster interactions can be ensured over zeolite H beta framework.