The effect of the phase composition of alumina supported NiMo catalytic precursors on thiophene hydrodesulfurization (HDS) was investigated. The catalytic precursors were prepared by impregnation of the commercial γ-Al₂O₃ with solutions of Anderson-type ammonium salts or co-precipitation of ammonium heptamolybdate and nickel nitrate. The precursors were characterized by XRD, BET specific surface area, pore volume and pore size, XPS, elemental analysis, TGA and ²⁷Al MAS NMR. The chemical analyses by ICP showed for the NiMo-AP compounds a clear agreement between experimental and theoretical values according to stoichiometric values (Mo/Ni = 6), while for NiMo-COP deviations were observed (Mo/Ni ∼ 7). The specific surface area and pore volume of NiMo-AP/γ-Al₂O₃ precursors were greater than those of the NiMo-COP/γ-Al₂O₃ precursors, 387/325 m² g⁻¹ vs. 283/265 m² g⁻¹, and 0.34/0.27 cm³ g⁻¹ vs. 0.21/0.15 cm³ g⁻¹, respectively; whereas the average pore radius for all systems was 12 Å. XRD and XPS analysis confirmed the presence of (NH₄)₄[NiMo₆O₂₄H₆]·5H₂O and Mo⁵⁺/Mo⁶⁺ for solids obtained by Anderson-type precursors, whereas NiMo-COP/γ-Al₂O₃ precursors exhibited Mo⁶⁺ from NiMoO₄ and MoO₃. The NiMo precursor obtained from conventional methods showed a higher amount of sulfur than those synthesized from the Anderson-type phase (6.9 to 4.9 wt%), although this does not mean a highly active sample or optimum sulfided active phase. ²⁷Al solid-state MAS NMR showed higher tetrahedrally coordinated aluminium for the NiMo-COP/γ-Al₂O₃ catalytic precursors. The catalytic activity was strongly influenced by the type of catalytic precursor and metallic wt%. The activity of the catalysts obtained by the sulfided Anderson-type ammonium salts was greater than the sulfided solids obtained by the conventional method, suggesting that these precursors result in a better active phase with a molar ratio (Ni + Mo)/S = 1.01 (likely “Ni–Mo–S” species), due to lower loss of the Ni promoter into the alumina support (²⁷Al NMR) and the lowest metal–support interaction (TGA). The catalysts obtained the HDS products, butane and cis-butene independent of the precursor type. Furthermore, the catalysts with 15 wt% Mo were more efficient than those obtained with 8 wt% Mo.