The potential of carbon and sulfur isotope ratios to group crude oils with respect to their origin was investigated. Sample selection was based on the actual crude oil imports to Germany. Analyzed crude oils from Algeria, the Community of Independent States (CIS), Middle East, Nigeria, the North Sea and Venezuela make up over 86% of the German crude oil imports. The oil as received was deasphalted and the maltene fraction was sepd. by MPLC into satd., arom. and polar fractions. Due to overlapping areas, it is not possible to group the crude oils by their d13C values alone. A complete grouping of the crude oils with respect to their origin can only be achieved by the combined use of d13C and d34S of crude oils, and isotope type-curves. In some cases isotope type-curves enable differentiation between different oil fields of the same geog. origin. In order to det. the post-spill changes of d34S values, an exptl. spill of crude oil was studied over a period of seven weeks in an outdoor aquarium contg. pond water. The d13C measurements of crude oil fractions showed changes up to 1,1 .permill. during the oil spill simulation. The d13C values of the polar fraction exhibited the smallest change, with a variation of 0,3 .permill., and are therefore esp. useful for the characterization of crude oil spills.