This work combines the outcomes from the electron spin resonance (ESR) analysis of crude oils with the results of their chemical composition from elemental analysis (CHN), gas chromatography with flame ionization detection, Fourier-transform ion cyclotron resonance mass spectrometry using electrospray ionization in negative mode [ESI(-) FT-ICR MS], and Fouriertransform infrared spectroscopy analyses, searching for correlations to identify paramagnetic constituents associated with the presence of vanadyl species and free radicals in three Brazilian oils and in their resin and asphaltene fractions. The ESR analysis of the light oil SO02 (43 degrees API) was able to differentiate it from biodegraded oils C19 (25 degrees API) and C10 (19 degrees API), through the quantification of free radicals in C19 (10.60 au) and C10 (9.39 au) when compared to SO02 (6.16 au), as a result of the electron paramagnetic resonance present in compounds that make up their polar fractions. The ESR response of the C19 resin fraction was the one that came closest to the oil's response, probably due to the greater relative abundance of compounds of N and NO classes in the C19 oil, observed through the results of the FT-ICR MS analysis. The ESR technique was also able to differentiate the biodegraded oils, due to the higher response of the vanadyl species in the C19 oil when compared to the C10 oil. O-2 compounds (carboxylic acids) present in high abundance in the C10 oil confirmed that it had undergone more extensive biodegradation than the C19 oil, and we suggest that this oxidation process may have decreased the response of the vanadyl species signal. The knowledge of the existing correlation between the response of the ESR technique of oils and their chemical composition aims at the application of this technique in the oil characterization during exploration, production, and refining operations.