The influence of P concentration on the synergism between Co/gamma-Al2O3 or Ni/gamma-Al2O3 and Mo/gamma-Al2O3 stacked beds separated by a 3-mm stack of P-x/gamma-Al2O3 is studied in the hydrodesulphurization of gasoil under operating conditions similar to those of industry. The results shown increased Co//Mo and Ni//Mo synergism related to changes in zero point of charge (ZPC), acid strength, and band intensities of diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy due to the incorporation of P into the gamma-Al2O3 used as separator. (C) 2009 Elsevier B.V. All rights reserved.