A deactivation study during hydroprocessing of Maya heavy crude oil is reported. Experiments were carried out in fixed-bed pilot plant at the following constant operating conditions: pressure of 70 Kg/cm(2), Hydrogen-to-oil ratio of 5000 ft(3)/bbl, space velocity of 1.0 h(-1), and reaction temperature of 400 degreesC. Deactivation of catalyst was followed by monitoring the changes in asphaltenes, sulfur, metals (V and Ni), Rambottom carbon, and nitrogen contents in the products. The catalyst was characterized before and after reaction. Results indicate that catalyst deactivation is very fast during the first hours of the run. Coke and metals depositions were around 18 and 10%, respectively. Accumulation of vanadium was higher in the lobes of the catalyst.