The effect of hydrogen purity on hydroprocessing of heavy gas oil (HGO) was studied in a trickle bed reactor over a commercial Ni-Mo/gamma-alumina catalyst. Methane was used as a diluent for the hydrogen stream, and its effect on the catalyst performance was compared to that of helium, which is inert toward the catalyst. Furthermore, a deactivation study was conducted over a period of 66 days, during which the catalyst was subjected to H-2 purity ranging from 95 to 75% (with the rest methane); no significant deterioration in the hydroprocessing activities of the catalyst was observed. Therefore, it was concluded that methane was inert toward a commercial Ni-Mo/gamma-alumina catalyst. However, its presence resulted in hydrogen partial pressure reduction, which in turn led to a decrease in hydrodesulphurization (HDS), hydrodenitrogenation (HDN), hydrodearomatization (HDA) conversions. This reduction can be offset by increasing the total pressure of the system. HDS, HDN, HDA, and mild hydrocracking (MHC) conversions were studied. Also determined were cetane index, density, aniline point, diesel index, and fractional distribution of the products. The temperature and liquid hourly space velocity (LHSV) were set constant at 380 degrees C and 1 h(-1), respectively. Pressure and gas/oil ratio ranges were 7.2-10.1 MPa and 400-1270 mL/mL, respectively. The hydrogen purity was varied in the range of 0-100 vol % (with the rest methane or helium).