The potential of electron paramagnetic resonance (EPR) spectroscopy to provide structural information on vanadyl species in asphaltene fractions was evaluated. The evolution of the chemical environment of vanadium during the fixed bed hydrotreating process was studied. This study shows that asphaltenes are composed of two types of environment: porphyrinic and non-porphyrinic structures. The hydroconversion process reduced by 40% the participation of non-porphyrinic species, while the porphyrinic complexes only diminished by 15%, which demonstrates that porphyrinic species are the most refractory to hydroconversion.