This contribution deals about the more relevant technical aspects to be considered when scaling hydrotreating catalysts based on CoMoP/gamma-Al2O3 and NiMoP/gamma-Al2O3. The effect of porous structure of the alumina support, the impregnating solution properties, the aging time and drying conditions on the texture, surface dispersion of the active phase, elemental radial distribution profile and catalytic activity properties were investigated. When using high CoMoP solution concentration, the rate of diffusion intra- and extra-granules were influenced by the high density, viscosity and liquid-solid contact angle and the pore structure of the catalyst support. Under this catalyst preparation conditions, lower capillary pressure and longer impregnation time values were obtained. This solution was unstable over time which represent a limitation for the catalyst commercial production. The catalyst also showed lower surface area and pore volume values and a non-uniform elemental radial distribution profile. The co-impregnation with diluted metal solutions in two successive steps was found to be the best procedure to prepare CoMoP/gamma-Al2O3 and NiMoP/gamma-Al2O3 catalysts at large scale when using the pore filling method. The catalysts showed higher surface area and pore volume and uniform elemental radial distribution profile. The aging time resulted as a key controlling parameter, thus the material obtained without aging showed a non-uniform Mo and P radial distribution profile and resulted in a less active catalyst. The drying step was also a critical preparation parameter to control; when the impregnated material was submitted to a fast drying, a metal composition loss was observed. The control of uniform metal deposition profile and the adequate alumina support selection were key factors for the successful scaling-up preparation process of highly active and selective CoMoP/gamma-Al2O3 and NiMoP/gamma-Al2O3 hydrotreating and mild-hydrocracking catalysts.