Thirty crude oils were sampled in the South German Molasse basin. The crude oils were chem. sepd. into 5 component groups and their stable C isotopic ratios were detd. by mass spectrometry. A differentiation of 4 regionally connected groups was inferred from the d13C values and chem. analyses. This grouping accorded with information on the origin of crude oils in the Molasse basin. The chem. of crude oils was influenced by secondary processes in the eastern part of the Molasse basin.