The most abundant metals in heavy feedstocks, vanadium and nickel, are mainly concentrated in the asphaltene fraction, a petroleum fraction that precipitates in the presence of paraffinic solvents. Characterization of vanadium and nickel complexes is therefore important to the development of demetalation and conversion strategies used to process heavy crude oils. The dependence of vanadyl structures upon the geographic origin of feedstocks and their evolution during hydroprocessing in an ebullated-bed pilot unit were studied. The aim of this contribution is to assess the possibilities of the electron paramagnetic resonance (EPR) spectroscopy to provide information on the structure of the vanadyl species. This work shows that pulsed ER spectroscopy is a powerful technique that allows us to distinguish several types of environments of vanadium species, among which are porphyrinic ligands, even in very complex samples, such as C-7 asphaltenes, from heavy feedstocks.