During the heavy oil hydroprocessing, a decrease in catalytic activity with time-on-stream (TOS), is due to the metal and carbon deposition on the catalysts. In this investigation, spent catalysts were obtained from the Maya heavy crude hydrotreating unit, as the processing conditions were close to the industrial practice. The catalysts were unloaded after considerable activity decrease, considering the initial activity as base activity. The spent catalysts were characterized by pore size distribution, SEM-EDS, TEM and C-13 solid state NMR. Results indicate that catalysts were deactivated mainly due to the deposition of metal and carbon. The sources of deactivation were identified along with the location of foreign species by analyzing the deposited species on the spent catalysts. The nature of the coke and its compositions were derived from C-13 CP/MAS and CP/MAS with dipolar dephasing NMR experiments. The wide range of analysis of spent catalysts specify that the catalyst pores were plugged during the heavy oil processing. The deactivation of catalysts were not only due to the carbon deposition but also due to the deposited metal sulfide (VxSy, NixSy) over the existing (CoMoS) active sites. It appear from the results that considerable amount of deactivation is irreversible and non-regeneratable (i.e., metal deposition). Hence catalyst deactivation during heavy oil hydrotreating (HDT) is a function of metal and carbon deposition during time-on-stream. The metal deposition on spent catalysts, e.g. vanadium is on the surface, while Ni distributed deeper into the pore of the catalysts. The removal of Ni and V showed different behavior, which was attributed to an oxygen atom presence to vanadium and its influence on the vanadium porphyrin structure. (C) 2013 Elsevier B.V. All rights reserved.