Hydrodesulphurization (HDS) kinetics of a straight run heavy gas oil has been studied using data collected from a trickle bed microreactor loaded with a diluted bed of a commercial catalyst. The reactor performance is simulated by a model taking into account the gas-liquid phase equilibrium. The gas oil compounds as well as the sulphur bearing compounds are represented by five pseudo-components. The sulphur removal and the hydrogen consumption are described by a set of parallel reactions of first order with respect to each pseudo-compound concentration. The reactants present in the liquid phase are considered to participate in catalytic reactions. (C) 2003 Elsevier Science B.V. All rights reserved.