The catalytic activities of several iron oxides and oxide hydroxides have been tested in the reduction of substituted nitroarenes using hydrazine hydrate as reducing agent. An iron oxide catalyst, prepared by rapid precipitation of iron hydroxides from solutions of iron(III)chloride, showed the highest activity mainly due to its large BET surface area compared to other iron oxides. Mossbauer spectroscopy and X-ray powder diffraction showed this catalyst to be ferrihydrite, a poorly ordered iron oxide hydroxide. During in situ experiments, in the absence of a nitrocompound, the complete transformation of ferrihydrite to Fe3O4 (magnetite) was observed. After addition of 4-nitrotoluene the catalyst was reoxidized to gamma-Fe2O3 (maghemite). The same experiment carried out in the presence of 4-nitrotoluene showed that after filtration and drying the catalyst still remained ferrihydrite. Thus, reduction of the catalyst by hydrazine hydrate and subsequent reoxidation by 4-nitrotoluene seems to occur only at the surface of the catalyst. No deactivation was observed after eight times recycling of the catalyst. Carrying out the reduction at 328 K, one could observe an increase in particle size. Increasing the reaction temperature from 328 to 373 K caused a successive increase in particle size of the ferrihydrite catalyst as well as the formation of alpha-Fe2O3 (hematite) which led to a significant decrease in activity. During the filtration procedure in air, the reduced catalyst showed pyrophoric behaviour that led to the formation of alpha-Fe2O3 (hematite).