Literature data covering the preparation and catalytic properties of Ir sulfide catalysts during the last two decades in relation to hydrotreating reactions, mainly hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of model compounds, have been reviewed. It has been shown that Ir sulfide belongs to the most active members of the third row noble metal sulfides. The specific activity of a well dispersed Ir sulfide in reactions of model compounds (type of thiophene, indole, pyridine, quinoline) is substantially greater than that of conventional CoMo (NiMo) catalysts. Sulfided Ir in an amount of about 0.5 wt.% is an efficient promoter of the Mo/Al2O3 catalysts, yielding synergetic effects in HDS and HDN of about 3. However, this synergy diminishes at the higher Ir loadings because of the drop in Ir dispersion. Modification of the NiMo/Al2O3 catalysts by Ir usually led to the decrease of HDS activity and the increase of HDN activity. (C) 2007 Elsevier B.V. All rights reserved.