A review. Compd.-specific stable isotope anal. (CSIA) using gas chromatog.-isotope ratio mass spectrometry (GC/IRMS) has developed into a mature anal. method in many application areas over the last decade. This is in particular true for C isotope anal., whereas measurements of the other elements amenable to CSIA (H, N, O) are much less routine. In environmental sciences, successful applications to date include (i) the allocation of contaminant sources on a local, regional, and global scale, (ii) the identification and quantification of (bio)transformation reactions on scales ranging from batch expts. to contaminated field sites, and (iii) the characterization of elementary reaction mechanisms that govern product formation. These three application areas are discussed. The studied spectrum of compds. comprises mainly n-alkanes, monoaroms. such as benzene and toluene, methyltert-Bu ether (MTBE), polycyclic arom. hydrocarbons (PAHs), and chlorinated hydrocarbons such as tetrachloromethane, trichloroethylene, and polychlorinated biphenyls (PCBs). Future research directions are primarily set by the state of the art in anal. instrumentation and method development. Approaches to use HPLC sepn. in CSIA, the enhancement of sensitivity of CSIA to allow field studies in the mg L-1 range, and the development of methods for CSIA of other elements are reviewed. Also, an alternative scheme to evaluate isotope data is outlined that would enable ests. of position-specific kinetic isotope effects and, thus, allow one to ext. mechanistic chem. and biochem. information.