The same chemical substance produced by different manufacturers can have a different isotopic composition. Therefore, stable isotope analysis of contaminants can be used to distinguish between different sources of contamination. Furthermore, during degradation processes, molecules with light isotopes are degraded preferentially. As a result, the heavy isotopes accumulate in the remaining fractionation of contaminants. Hence, stable isotope analysis can also be used to follow the progress of reactive processes. However, for contaminant sources located in the vadose zone between soil and groundwater table, isotope ratios can also be modify by physico-chemical processes. For example, during diffusion, molecules with light isotopes migrate more rapidly than molecules with heavy isotopes, leading to a depletion of heavy isotopes with distance. Hence when applying isotope methods to differentiate contaminant sources or to trace biodegradation, the effect of physico-chemical processes such as vaporization, air-water partitioning or diffusion needs to be quantified. In this project, the effects of these processes on isotope ratios are investigated in detail using chlorinated hydrocarbons, which are common environmental contaminants, as model compounds. The project includes batch experiment to investigate the effect of partitioning of compounds between different phases (organic liquid, water, air), column experiment to evaluate the effect of transport processes and field studies. The project will provide insight into mechanisms of isotope fractionation and a solid basis for the application of isotope methods to study the origin and fate of contaminants in the vadose zone.