The effect of a hydroxyl substituent on the hydrodesulfurization (HDS) of mercapto and methylmercaptobenzene was studied using a commercial presulfided CoMo/Al2O3 catalyst in a batch reactor under hydrotreating conditions (160-280 degrees C, 7 MPa). Mercaptobenzene, (methylmercapto)benzene, phenol, 4-mercaptophenol and 4-(methylmercapto)phenol were used as model compounds. The reaction data was evaluated using a pseudo first-order rate model. Thermal reactions of the model compounds were tested separately. The rates of HDS of the mercapto and the methylmercapto substituent were strongly affected by the presence of a hydroxyl substituent in the para position. At 200 degrees C, the HDS reaction rates of 4-mercaptophenol and 4-(methylmercapto)phenol were respectively 4.5 and 3.0 times higher than those of the corresponding oxygen-free compounds. The activation energies of HDS followed the order of the electronegativity of the sulfur atom; the activation energy was about 15% higher for the oxygen-containing compounds, and the methylation of the mercapto group increased the energies further by about the same amount. Thermal reactions of the mercapto group were also significant in the presence of the hydroxyl substituent. HDO of the phenolic hydroxyl was, on the other hand, suppressed in the presence of sulfur-containing substituents.