Although it is widely believed that production of organic compounds by Fischer-Tropsch synthesis and related processes occurs in many geologic environments, unambiguous identification of compounds with an abiotic origin in natural samples has been hampered by a lack of means to discriminate between abiotic compounds and organic matter from biological sources. While isotopic compositions might provide a means to discriminate between biologic and non-biologic sources of organic matter, there are few data presently available to constrain the isotopic composition of compounds produced by abiotic processes in geologic systems. Here, we report results of laboratory experiments conducted to evaluate the isotopic composition of organic compounds synthesized abiotically under hydrothermal conditions. We find the organic products are depleted in C-13 to a degree typically ascribed to biological processes, indicating that carbon isotopic composition may not be a particularly effective diagnostic means to differentiate between biologic and non-biologic sources. Furthermore, our results suggest that the isotopic compositions of reduced carbon compounds found in many ancient rocks that have heretofore been attributed to biological sources could be consistent with an abiotic origin in a hydrothermal setting. (c) 2006 Elsevier B.V. All rights reserved.