Ir/Al2O3 catalysts with high metallic contents are applied on satellite thruster to decompose hydrazine. The present work has as principal aim the study of the morphologic evolution of Ir/Al2O3 catalysts with metallic contents from 12 to 30 wt.%. The C catalysts were prepared through consecutive impregnations from the H2IrCl6 precursor, using three different types of aluminas. The specific surface area, volume and distribution of pore size, specific metallic area and metallic panicles average diameter. as well as the mechanical resistance were determined. Results show that the Ir addition leads to a decrease of the specific surface area and the pores volumes, while increases the mechanical resistance. Values for average diameter of metallic particles are comprised between 1.4 and 2.4 nm when the metallic content increases from 12 to 30 wt.%. Catalysts containing 30 wt.% of Ir presents specific metallic areas around 30 m(2)/g, although pores volumes and distributions of pore size were considerably different for the three supports. Their metallic particles dispersion and size values are very close to those of a commercial catalyst Shell 405, even though the preparation methods were different. These results show that there is a strong interaction between the alumina and the iridium precursor. (C) 2004 Elsevier B.V. All rights reserved.