Heavy feed upgrading in the atmospheric residues (AR) desulfurizatrion process suffers from rapid catalyst deactivation by coke deposition. The main source for the coke is unconverted, refractory asphaltenes that tend to coagulate and precipitate on the catalyst surface due to incompatibility with the hydroprocessed feed. Asphaltene fractions separated from three ARs, namely Kuwait export crude (KEC), Kuwait heavy crude (KHC), and Eocene (EOC), with an API gravity, of 13.5 degrees, 8.3 degrees, and 7.4 degrees, respectively, were pyrolised at 412 degrees C for 7 h under nitrogen. KEC-AR asphaltenes show the highest weight loss (26 wt %) followed by KHC-AR (21 wt %) and EOC-AR (18 wt %). KHC-AR (14 wt %) and EOC-AR (27 wt %) contain a significantly higher portion of refractory asphaltenes than KEC-AR (3 wt %). Applying the first- or second-order kinetic model, the rate constants of asphaltene conversion reflect this trend. Pilot plant tests revealed that KHC-AR and EOC-AR shorten the lifetime of a graded catalyst system typically used in a Kuwait refinery by 50% compared to KEC-AR. The structural changes in the asphaltenes occurring during pyrolysis were studied by solid-state C-13 NMR and X-ray diffraction. Under pyrolysis, more scissioning of side chains occurs than cracking in a more distant position from the aromatic ring. Dealkylation/ hydrogenation is preferred over formation of larger polyaromatics. As a consequence, the average distance between the aromatic sheets and the stack height of the aromatic sheets in asphaltene aggregates reduce while the average diameter of the aromatic sheets hardly changes.