The behavior of catalysts containing rhodium was evaluated in hydrotreating (HDT) reactions at 583 K and 5 MPa. The influence of the temperature and the nature of the activating agent were analyzed for the monometallic Rh/gamma-Al2O3 catalyst in simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of naphthalene (NT). H-2, H2S, N-2 and H2S (15 vol.%)/H-2, H2S (85 vol.%)/H-2 mixtures were used as activating agents. Bimetallic RhMo/gamma-Al2O3 catalysts with different Rh/Mo ratios were also tested in the same reactions. Some of the catalysts were characterized by XPS. The results show that for the Rh monometallic catalyst, the H2S present in the activating mixture leads to an increase in the selectivity to HDS. Synergy in both HDS and HYD reactions was detected for the bimetallic catalysts, and it presents a slight increase with the Rh/(Rh + Mo) atomic ratio. These results lead to speculate about the possible role of Rh in bimetallic catalysts. (C) 2007 Elsevier B.V. All rights reserved.