The distribution of coke inside the pellets of several industrial spent hydroprocessing catalysts was analyzed using Raman spectroscopy. The shape of the coke profiles gives us information on the deactivation mechanism and the magnitude of diffusion limitations. Samples taken at the end of run showed the most pronounced coke profiles. Typical M-shaped profiles observed in a guard bed catalyst suggest a sequential deactivation mechanism and a strong diffusion resistance. Consequently, it is concluded that near the end of life the catalyst effectiveness factor can be significantly reduced by coke deposition. (c) 2006 Elsevier Ltd. All rights reserved.