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Measuring stable isotope fractionation of carbon, hydrogen, and other elements by Compound Specific Isotope Analysis (CSIA) is a new, innovative approach to assess organic pollutant degradation in the environment. Central to this concept is the Rayleigh equation which relates degradation-induced decreases in concentrations directly to concomitant changes in bulk (= average over the whole compound) isotope ratios. The extent of in situ transformation may therefore be inferred from measured isotope ratios in field samples, provided that an appropriate enrichment factor (ε_bulk) is known. This ε_bulk value, however, is usually only valid for a specific compound and for specific degradation conditions. Therefore, a direct comparison of ε_bulk values for different compounds and for different types of reactions has in general not been feasible. In addition, it is often uncertain how robust and reproducible ε_bulk values are …