Hydroprocessing is one of the key unit operations in today s oil refineries. Its importance will increase in the coming decades, due to more stringent fuel quality legislation and further growth of the global fuel demand. Moreover, nitrogen compounds, naturally occurring in crude oil, have been identi ed as strong inhibitors of the hydrodesulfurization (HDS) reactions during the hydrotreatment process. This project was divided into two phases: in phase one a parametric and kinetic study of HDS activity for an oil fraction loaded with thiophen, as a sulfiding model, onto a commercial CoMo/Al2O3 were investigated, and in phase two the effect of two different nitrogen compounds (e.g., carbazole and pyridine), which are commonly identified in fractions of crude oil, on the HDS activity and hydrodenitrogenation (HDN) activity was studied. A kinetic analysis of the operating system was also included in phase two.