A diesel fuel desulfurized to 76 and 13 ppm sulfur in the aromatic fraction was analyzed with the objective of performing a speciation of the polycyclic aromatic sulfur heterocycles present. This provides information of importance to catalyst and process development. The analysis was based on selectively separating the sulfur from the non-sulfur aromatic compounds on a PdII- containing liquid chromatographic phase, followed by identification and quantification using gas chromatography with mass- selective and sulfur- selective atomic emission detection. Several newly synthesized alkyldibenzothiophenes ( mainly C-3- and C4DBTs) could be quantified, and isomers with up to six alkyl carbon atoms could be detected in the diesel fuel. Besides confirmation that substituents in the 4 position of a dibenzothiophene lead to low rates of hydrodesulfurization ( HDS), it is reported for the first time that a methyl group in the 1 position can also lead to a lower reactivity under HDS conditions, allowing such compounds to accumulate in the deeply desulfurized products. It is suggested that the nonplanarity of the aromatic system in such compounds is associated with this phenomenon.