The initial activity of CoMo/Al2O3 and NiW/Al2O3 hydrodesulfurization catalysts and influence of the reaction conditions on activity were investigated. The set of hydrodesulphurization experiments at three different temperatures and three different hydrogen partial pressures was performed. Experimental data were evaluated by nonlinear regression. Furthermore, two mathematical models were defined. The first one in Aspen Plus, using pseudo-first order reaction kinetic, including solubility of hydrogen in liquid. The second one in Matlab computing pseudo-first reaction order and reaction order 0.75. This model is describing research area more precisely.