Different severities of hydrotreatment were performed on a raw decant oil and one of its subsequent hydrotreated decant oils. Six hydrotreated decant oils with different chemical composition were obtained during this process. The products obtained from the raw decant oil showed difficulty in sulfur removal. Higher desulfurization was obtained after repassing the first drum of product (EI-132) at a reduced feed rate (LHSV 0.5h(-1)), higher temperature (734 degrees F and 750 degrees F), and higher reactor pressure (8.3 MPa). Several analyses were performed to characterize the decant oils: elemental analysis, H-1 and C-13 NMR, asphaltene content, average boiling point, API gravity, GC/MS, and viscosity. Two structural parameters derived from the H-1 NMR spectra and elemental analyses were calculated: aromaticity (fa) and the fraction of aromatic edge carbons carrying substituents (sigma). All the information gathered from the different techniques was helpful to understand the chemical transformations taking place during hydrotreatment of decant oils. It was observed that hydrogenation of aromatic rings and hydrodealkylation were the dominant reactions in the severely hydrotreated decant oils (EI-137 and EI-138) while the formation of alkyl side chains was observed in the low and intermediate hydrotreated decant oils, (EI-133 and EI-134) and (EI-135 and EI-136), respectively.