A study of vanadyl-porphyrin by electron spin resonance (ESR) was carried out looking for answers about the role that the central V=O ion plays when these kinds of molecules are present in heavy crude oils. The eigenvalues of the linear combination of atomic orbitals (LCAO) were obtained from the experimental values of g and parameters (ESR). The contributions to the molecular orbitals that describe the various energy levels of vanadyl ion were also obtained for the porphyrin species. The trends of the degree of covalent character of the metal-ligand bonds and the length of the vanadium-oxygen chemical bond are discussed. It is interesting to note that the Fermi contact term, K-eff, is essentially constant for all samples investigated, and it was found to be independent of the calculated electron delocalization (I - delta (2)) and shows only little variation among the three different samples of oil. The orbital energies derived from our ESR study qualitatively agree with those predicted from MO theory for synthetic vanadyl-porphyrins.