Crude oils usually show relatively intense ESR signals. The signals depend on the type and the origin of oil. Two common components in crude oil are: a signal from some vanadium admixture and an organic radical. The organic signal is most intense in heavy oils containing asphaltenes. It directly reports on the asphaltene content in oil and its propensity to form carbon residue. More detailed analysis of the organic signal also can report on degradation and modification of oil. The vanadium signal varies depending on the oil origin, whereas being also pronounced in heavy oils. After oil extraction the intensity of the organic signal usually decreases with time, while the vanadium signal does not change. ESR can characterize these components as well as for the content of some other ESR active compounds (e.g. nickel).