The present work is focused on attempt to understand the importance of aromaticity in the elimination of N, O and S atoms from five-membered heterocycles and catalyst poisoning by basic nitrogen six-membered heterocyclic compounds in the hydrotreatment (HDT) process. Pyrrole, furan, thiophene and pyridine, and their fused benzene ring derivatives have been studied by means of density functional theory (DFT). Proton affinities (PAs) were calculated for all these compounds, geometrical modifications were observed during protonation. Five-membered aromatic compounds break planarity due to proton interaction of the heteroatom, while pyridine-type compounds do not. Aromatic stabilization energy (ASE) was calculated by homodesmotic chemical reactions for all heterocyclic compounds. The energetic trends for protonated and unprotonated compounds confirm the less reactivity of dibenzo-compounds in the HDT process and catalyst's poisoning by pyridine derivatives present in crude oils. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.