A model NiMo/Al2O3 catalyst was aged under various process conditions during the hydroprocessing of light gas oil. In order to enhance coke deposition, experiments were done at high temperature (573-673 K), low pressure (1-5 MPa) and hydrogen to oil (HTO) ratio (190-560 Nm(H2)(3)/m(oil)(3)), and with the addition of polyaromatic compounds. No catalyst deactivation was observed during the initial 50 h on stream. At high HTO ratio a moderate coke deposition was observed, almost independent of pressure and temperature. At low HTO ratio, the conversion decreased and the amount of coke increased, due to hydrogen depletion. At low HTO ratio, the addition of polyaromatics lowered the HDS conversion, which was found not to be due to inhibition but rather to competition in hydrogen consumption. Aromatics addition did not enhance coking, however. During aging, a significant fraction of the catalyst initial pore volume was blocked by coke deposits. Calculations predicted no diffusion limitations for the aromatic and sulfur-containing reactants but significant hydrogen concentration gradients inside the catalyst pellets at low pressure. This mass transfer limitation of hydrogen resulted in the formation of typical bell-shaped coke deposition profiles, which could be explained by a simple model assuming the rate of coke deposition being inversely proportional to the hydrogen partial pressure.