The effect of reaction/regeneration cycles on some structural characteristics of a NiMo/Al2O3 hydrotreatment catalyst has been studied. The first regeneration produces increases in pore volume, porosity, and specific area. Above 5% coke deposit, various changes in such properties appear. Mo and Ni relative XPS intensities depend on the reaction/regeneration cycles Ni exposure decreases with them, whereas Mo exposure follows the opposite trend. The effect of coke on metal dispersion seems to be almost negligible. Coke concentration proved to be the variable which really influenced coke composition (H/C), independently of the history of the catalyst. The equation relating both variables is H/C = 0.4378 + 1.342 exp[(-C-o/S-e)/0.059] where C-c is the percentage coke concentration and S-e the specific surface area.