Deep hydrodesulfurization (HDS) of petroleum feedstocks is becoming more important and necessary worldwide as governments reduce further sulfur levels diesel and gasoline fuels. After initial hydrotreatment, the main refractory sulfur compounds remaining are dibezothiophene (DBT) derivatives which make deep hydrodesulfurization difficult. The influence of nitrogen compounds and aromatic species on HDS catalyst activity becomes more significant under deep hydrodesulfurization conditions. It is of great interest to have a better knowledge of the binding of refractory sulfur compounds and other aromatic components, such as nitrogen compounds and polyaromatic species, to metal sites on the catalyst. Equilibrium constants for the displacement of DBT in [CpRu(η6-DBT)](PF6) with a series of nitrogen cyclic compounds and polyaromatic species (L) were measured in order to understand the inhibitory effects of nitrogen and aromatic species on hydrodesulfurization reactions. The relative binding strengths decrease in the order mesitylene > toluene > (3-Me)indole - tetrahydronaphthalene > indole > carbazole - (3-Et)carbazole > benzene > fluorene - (3,6-(t-Bu)2)carbazole > biphenyl > (4,6-Et2)DBT > phenanthrene > (2-Me)naphthalene - (1-Me)naphthalene - naphthalene > (3,6-Br2)carbazole. These results will be discussed in the context of their tendencies to inhibit HDS reactions.