The effect of trimesic acid (TA) on the surface of γ-Al2O3 for CoMoP catalysts has been investigated in the hydrodesulfurization (HDS) of straight-run gas oil. The γ-Al2O3 support was modified by surface impregnation of a solution of TA to afford TA/Co molar ratios (0.5, 1.0 and 2.0) in the final composition. The CoMoP material with atomic ratio of TA/Co = 1.0 displayed the highest HDS activity. FT-IR results suggest that the carboxylic acid moieties (AA) of the TA deposited on the γ-Al2O3 interact with the Mo and Co species. The DRS UV vis revealed the presence of Co species in distorted octahedral symmetry. The MoOh/MoTh ratio varied depending on the amount of the organic additive. TGA results indicated that the TA on the catalyst with TA/Co ratio = 1.0 often decomposes at lower temperature in comparison with the other samples. Thus, a lower temperature decomposition favors the surface concentration of metals species for the sulfurization degree. The interaction of the TA induces moderate metal-support interaction. Raman spectroscopy showed that the presence of TA influenced significantly on the ratio Mo[dbnd]O/(Mo[dbnd]O + Mo–O–Mo + Mo–O) on the catalyst surface. Raman results also evidenced the highest ratio Mo[dbnd]O/(Mo[dbnd]O + Mo–O–Mo + Mo–O) for the CoMoP (1.0) sample. Apparently, the augment of the Mo[dbnd]O species is related to a major availability of Mo species for the formation of MoS2 for HDS reaction. The XPS and HRTEM results showed higher formation and more dispersed MoS2 and CoMoS species for the catalysts modified by TA compared to that of the reference sample. The CoMoP (1.0) showed higher Moe/Mot ratio in comparison with CoMoP (0.0), which resulted in a higher gas oil HDS activity.