Hydrotreating (HDT) of model molecules over high specific surface area Co(Ni)Mo/MgO-Al2O3 mixed oxide supported catalysts have been investigated for thiophene HDS in a micro-reactor at 400 degreesC and atmospheric pressure. Thiophene catalytic activity was studied as a function of Mo content (2-16 wt.% Mo), promoter, and support composition [MgO/(MgO + Al2O3)x =0-0, 0.01, 0.05, 0.10, 0.50, 1.0]. Supported catalysts were characterized by BET-specific surface area, X-ray power diffraction (XRD), pore volume, and pore size distribution. The promoted molybdenum sulfided catalysts showed a wide range of activity variation with support composition, which establishes that molybdenum sulfided active phases strongly depend on the nature and composition of the support. Maximum activity was observed on sulfided catalysts at MgO/(MgO + Al2O3) = 0.05. With variation of Mo loading, the activities increase up to 10 wt.% molybdenum loading and after that activity remains constant with further increase in Mo content. The SSA, XRD, and pore volume characterization results are in good agreement with MoO3 monolayer formation. The incorporation of MgO with gamma-Al2O3 alters the nature of active phase interaction on the support surface. Therefore, MgO counterpart plays a structural promoting role to the support contribution and its interaction towards the active metal geometry. One lowest composition of MgO (I wt.% MgO) supported catalyst tested at high pressure with real feed (Maya heavy crude) was compared with a commercial catalyst. The lab prepared catalyst was found to be more stable as well as more active for hydrogenolysis [C-S, C-M (Ni, V)] functions, which were improved by using small amount of MgO with alumina. (C) 2004 Elsevier B.V. All rights reserved.