Hydrodesulfurization (HDS) of vacuum gas oil (VGO) was carried out in a trickle bed reactor at 400 degrees C and 7.2 MPa with a weight hourly space velocity (WHSV) = 2 h(-1) over a series of molybdenum-nickel catalysts, which were prepared by loading the same amounts of MoO3 (12 wt %) and NiO (3 wt %) on supports including: tubular shaped MCM-41, nontubular shaped MCM-41, alumina, and silica. The HDS activity of the MoNi catalysts with different supports was investigated. The tubular shaped MCM-41, which possesses a regularly ordered molecular sieve structure, was shown to give significant improvements in the diffusion of large molecules. The results indicated that the tubular shaped MCM-41 supported catalyst has a higher activity than the other catalysts when measured over a 4-5 day running period. The effect of Si/Al ratios in the framework is also discussed. Various methods and techniques were used to characterize the catalysts such as: scanning electron microscopy (SEM), transmission electronic microscopy (TEM), temperature programming reduction (TPR), X-ray diffraction (XRD), mercury-porosity, and the generation of nitrogen adsorption-desorption isotherms.