In the hydroprocessing of coker naphtha (hydrodesulfuration and hydrodenitrogenation), silica poisoning of hydrotreating catalysts represents a severe problem due to the deposition of Si species, which deactivate the catalyst and shorten its life time. The deactivation results of the sulfided NiMo/gamma-Al2O3 hydrotreating catalyst, obtained by running a long-term test, are presented. Refinery coker naphtha was processed in a continuous hydroprocessing unit in a mini-scale laboratory under industrial conditions. Catalyst samples from 15 to 90 days on stream were characterized. Si determination and the evolution of the structural characteristics of these samples were followed by Si-29 MAS NMR, FTIR, textural analysis and scanning electron microscopy (SEM) performing microanalysis by dispersion energies (EDS). Si-29 MAS NMR results confirmed the deposition and evolution of different Si species on the catalyst surface with the time on stream. The increasing content of Si species in the catalyst affected textural properties such as surface area and pore volume by decreasing them; as for the chemical properties, the Bronsted/Lewis (B/L) ratio was modified without increasing the coke formation and cracking activity. From the coker naphtha hydrotreating processes, only the hydrodenitrogenation activity was affected. The Si deposition profile shows that the concentrations were higher in the external than in the internal surface of the pellets. The shape of these profiles indicates that the deposition process becomes diffusion rate limited. (C) 2014 Elsevier B.V. All rights reserved.