Co-processing of gas oil and vegetable oils for the production of diesel-green diesel mixtures aims at producing deep hydrodesulphurized fuels at an acceptable cost. Gas phase analysis and data treatment can effectively provide knowledge for catalyst selectivity and hydrogen consumption. In this work, the reactor outlet gas composition was calculated from laboratory data and then the catalyst selectivity was estimated. Apart from CO and CO2, CH4 was also present in significant amounts in the effluent gas, indicating that methanation reaction occurs in the gas phase. The water gas shift reaction was found to be at equilibrium, which does not allow distinguishing the two reactions mechanisms, decarboxylation and decarbonylation. Deep hydrodesulfurization (HDS) was simulated by two sulfur pseudocomponents, one of high reactivity and one of low reactivity. Experiments were carried out in a bench-scale trickle-bed reactor at typical HDS conditions.