Volatile fatty acids (VFAs) are central intermediates of anaerobic metabolism and present in aquatic environments such as marine sediments. Conceptually, the isotopic composition of volatile fatty acids is presumed to be sensitive to the specific biogeochemical processes involved in their production and consumption. However, due to generally low environmental concentrations, our knowledge on isotopic variability of VFAs is limited. We report the development and application of a new protocol for compound-specific carbon isotopic analysis of VFAs in marine porewaters and other aqueous liquids. This new protocol involves reversed-phase separation of volatile fatty acids with an aqueous mobile phase by high performance liquid chromatography (HPLC) combined with chemical oxidation of the effluents by the Finnigan (super TM) LC IsoLink interface (Krummen et al. 2004) and subsequent online transfer of the resulting CO (sub 2) into an isotope ratio mass spectrometer. We obtained reproducible and accurate results for pure Na-acetate dissolved in artificial seawater at concentrations as low as 2 mu M, whereas minimum concentrations in marine porewaters were 10 mu M. Our technique extends the previously accessible concentration range and the fully automated online operation allows for systematic analysis of large sample sets. These technical improvements make carbon isotopic analysis of selected VFAs a realistic perspective for many sedimentary environments. Our initial survey of acetate in porewaters and fluids obtained from incubations of marine sediments has revealed an unexpectedly large range of isotopic compositions from -5 ppm to -85 ppm and provides strong support for process-specific information encoded in the isotopic compositions of VFAs.