The electronic structure and proton affinities of a series of organosulfur compounds such as thiophene, dibenzothiophene and a series of their derivatives have been studied by means density functional theory. The molecular structure of these compounds and their protonated species has been obtained by means of geometry optimization and characterized as potential energy surface minima by harmonic frequency analysis. The analysis of frontier orbitals, aromaticity and proton affinity permits one to make quantitative predictions about the difficulty to remove sulfur from these molecules. In particular, it is suggested that the proton affinities provide a descriptor of the activity of these molecules towards hydrodesulfurization. (C) 2007 Elsevier B.V. All rights reserved.