Industrial spent catalysts taken from different beds of industrial lube oil hydrotreating reactors were investigated in a pilot plant. Experiments to quantify the catalytic activity loss for hydrodesulfurization, hydrodenitrogenation, and aromatic hydrogenation reactions were carried out using an industrial feedstock under operational conditions similar to the industrial ones. Characterization of the spent catalysts was performed to demonstrate the relative influence of different possible deactivation causes. Depending on the position of the catalyst inside the industrial reactor, different deactivation causes were evaluated. At the top of the reactor, poisoning by silicon, arsenic, and iron was the main cause, while at the bottom, coke deposition was predominant.