A long term (7400 h) petroleum residue hydroprocessing study was conducted with a view to gain a clear understanding of the nature of the coke that deposits on the catalyst surface during the operation and to investigate the influence of processing time on coke build-up. Elemental analysis of the catalysts showed a rapid coke build-up on the catalyst surface during the early hours of the run, reaching as high as 12.4 wt.% within 100 h, and a maximum value between 1100 and 2100 h. From 2100 h, it was showed a significant decrease of the carbon content reaching a stable value from 3100 h. The enhanced nitrogen content of the coke deposits suggested that asphaltene fractions containing nitrogen were concentrated on the catalyst in the initial operation period. Additionally, the coke deposits on the spent catalysts were investigated using C-13 solid-state nuclear magnetic resonance techniques of cross-polarisation with magic-angle spinning (CP-MAS) for characterising the aromatic and aliphatic carbon types. Nuclear magnetic resonance (NMR) spectra revealed an increase of the aromaticity with a further increase of time on stream and the removal of aliphatic reversible carbon from the catalysts. Temperature-programmed oxidation (TPO) study showed the existence of different profiles, which characterised the reactivity of the deposits on the catalysts.