Ozonation products of sour crude oil have been determined at the molecular level by means of ion cyclotron resonance Fourier-transform mass spectrometry in soft ionization modes. It has been shown that a proper choice of ozonation conditions makes it possible to selectively ozonize heteroatomic nitrogen, sulfur, and oxygen compounds, such as pyrrole derivatives, sulfides, disulfides, thiophene derivatives, resins, and asphaltenes. It has been found that the hydrocarbon portion of the oil is not ozonized under the given conditions.