The influence of catalyst acidity on the reaction mechanism of the 4,6-dimethyldibenzothiophene (4,6-DMDBT) hydrodesulfurization (HDS) over CoMo hydrotreating catalysts was studied using a batch autoclave and fixed-bed reactor (T = 598 K and P = 5.5 MPa). P-free Ti-HMS and P-containing P/Ti-HMS mesoporous siliceous materials were synthesized and used as supports. The effect of the catalyst preparation method (successive vs. simultaneous impregnation) and phosphorous addition on catalyst acidity was studied by TPD-NH3 and DRIFT-NH3 techniques. For all synthesized catalysts, the reaction proceeds via dealkylation (DA) and isomerization (ISO) pathways, with the later being the main reaction route. Incorporation of Co and Mo phases by simultaneous impregnation was found to be the best method for catalyst preparation, whereas P-addition promote the isomerization route of 4,6-DMDBT transformation to a greater extent than the dealkylation route. On the contrary, the reaction on a CoMoP/gamma-Al2O3 reference catalyst proceeds via HYD and DDS reaction pathways, the later being the main reaction route. The catalyst acidity-activity correlation indicates that both activity and selectivity depend largely on the presence of Bronsted acid sites as well as on the total amount of Bronsted and Lewis acid sites. By correlating HRTEM-activity data, the enhancement of activity and isomerization observed with the catalyst prepared by simultaneous impregnation and modified with P was related to the cumulative effects of the lower size of MoS2 slabs and their higher surface density. (C) 2007 Elsevier B.V. All rights reserved.