Hydrotreating of heavy oil fractions is usually carried out in fixed bed reactors using alumina-supported Ni-Mo and Co-Mo catalysts. The catalytic properties of hybrid Al- and Ti-MCM-41 materials were compared for HDS of thiophene and heavy vacuum gas oil (HVGO) derived from Maya crude. Upon addition of 20 wt.% TiO2, the hexagonal symmetry of the pore arrays was unchanged, but the surface area increased from about 1040 up to 1138 m(2)/g. In parallel, NH3-thermal programmed desorption (TPD) studies showed an increase of surface acidity with TiO2 content, i.e., from 38 mumol NH3/h for initial MCM-41 to about 62 mumol NH3/g for 80% Ti-substituted Si-MCM-41. Also, Ni-Mo and Ni-W metal loadings increased the total surface acidity, from about 44 mumol NH3/g for NiMo/MCM-41 to about 148 mumol,NH3/g for NiMo/MCM-41. Also, the hydrothermal stability of the Ti-MCM-41 materials increased from about 500 degreesC upto 600 degreesC under 100% water vapor. Finally, sulfided forms of Ti-MCM-41-supported Ni-Mo and Ni-W catalysts were tested for HDS of heavy vacuum gas oil and thiophene at 350 degreesC, the NiMo/Ti-MCM-41 and NiW/Ti-MCM-41 catalysts with 50% Ti showing the highest activity, which was comparable to reference catalysts. (C) 2004 Elsevier B.V. All rights reserved.