An experimental study was conducted in a pilot plant fixed-bed reactor in order to evaluate the effect of hydrogen sulfide on hydrotreating activity of an industrial heavy gasoil Fluid Catalytic Cracking (FCC) feed recovered from a Mexican refinery. Hydrotreating reactions were carried out at three temperatures (603, 638 and 673 K), 11.76 MPa of pressure and LHSVof 6 h(-1). Studies were done varying hydrogen sulfide concentration 0, 100, 450 and 750 vol ppm (0, 3100, 12,960 and 23,210 Pa, respectively) at the same operating conditions over a commercial NiMo/gamma-Al2O3-TiO2 catalyst. Experimental results showed different behavior for these reactions. An inhibiting effect on hydrodesulfurization (HDS), hydrodearomatization (HDA) and hydrodenitrogenation (HDN) was observed, whereas the hydrodemetallization (HDM) reaction showed an increase in the activity. Apparent activation energies were also determined for each reaction. (c) 2005 Elsevier B.V. All rights reserved.