Pure gamma-alumina as support for CoMo sulfided catalysts is compared in terms of physical and catalytic hydrodesulfurization (HDS) properties to other supports. gamma-alumina containing foreign elements as impurities, i.e. Mg, Ca, Na, Fe, Cl, S and P does not exhibit significant changes. Other crystalline phases such as eta, theta, chi or amorphous aluminas as support show important decreases in HDS activity of the corresponding CoMo catalysts. This activity is linearly correlated to the mean pore diameter of the support. Other supports such as TiO2, ZrO2, MgO, CeO2 and SiO2 not only show lower activities but also changes in selectivity.