The influence of gallium on alumina-supported NiMo catalysts was investigated by correlating their physicochemical properties with the hydrodesulfurization (HDS) activity of model molecules. The Ga-gamma-Al2O3 Supports were prepared by impregnation of Ga (0.6-5.9 wt%) on gamma-Al2O3. The NiMo catalysts were prepared following the same method on calcined Ga-Al2O3. The N-2-physisorption results showed that Ga did not affect the textural properties of both gamma-Al2O3 and NiMo/gamma-Al2O3. When the Ga-Al2O3 Supports were calcined at 723 K all Ga ions presented a high dispersion, forming a different structure than that observed in bulk Ga2O3. The XPS and the TPR-S showed that the Ga addition modified the structural properties of NiMo depending on the amount of Ga. In the HDS of DBT and 4.6-DMDBT. the Ga-Al2O3 supported NiMo catalysts at loadings of Ga below to 1.2 wt% showed higher activity than that of a NiMo/Al2O3 sample. (c) 2005 Elsevier Inc. All rights reserved.