The effects of H(2) purity, pressure, gas/oil ratio, temperature, and LHSV on hydrotreating activities were investigated in a micro-trickle bed reactor using a commercial NiMo/gamma-Al(2)O(3) catalyst. Heavy gas oil (HGO) from Athabasca bitumen was used as feed. Due to their significant effects on H(2) partial pressure, H(2) purity, pressure, and gas/oil ratio were chosen and used in a central composite design (CCD) method. Experimental conditions used were H(2) purity, pressure, and gas/oil ratio were: 75-100 vol.% (with the rest methane), 7-11 MPa, and 400-1200 mL/mL, respectively. The effect of LHSV (0.65-2 h(-1)) and temperature (360-400 degrees C) were studied in a separate set of experiments. Vapor/liquid equilibrium (VLE) calculations were performed to determine the inlet and outlet H(2) partial pressures. It was observed that the enhancing effects of H(2) purity on hydrodenitrogenation (HDN) and hydrodearomatization (HDA) activities were greater than that of gas/oil ratio; however, it was comparable to pressure. Hydrodesulphurization (HDS) activity was not considerably affected by H(2) purity, pressure, or gas/oil ratio. Increasing LHSV led to a decrease in HDS, HDN, and HDA activities while increasing temperature resulted in an increase in HDS and HDN; HDA had maximum activity at about 385 degrees C. Kinetic fitting of the data to a pseudo-first-order power law model suggested that conclusions on hydrotreating activities' responses to a changing H(2) pressure could be equally drawn from either inlet or outlet H(2) partial pressure. However, from the catalyst deactivation standpoint, it is recommended that such conclusions are drawn from the outlet H(2) partial pressure. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.