A gas chromatography-single quadrupole mass spectrometry method was developed and validated for compoundspecific chlorine isotope analysis (Cl-CSIA) of three chlorinated herbicides, atrazine, acetochlor and metolachlor, which are widespread contaminants in the environment. For each compound, the two most abundant ions containing chlorine (202/200 for atrazine, 225/223 for acetochlor and 240/238 for metolachlor) and a dwell time of 30 ms were determined as optimized MS parameters. A Limit of Precise Isotope Analysis for ethyl acetate solutions of 10 mg/L atrazine, 10 mg/L acetochlor and 5 mg/L metolachlor could be reached with an associated uncertainty between 0.5 and 1‰. To this end, samples were measured tenfold and bracketed with two calibration standards which covered a wide range of δ 37 Cl values and whose amplitudes matched those of the samples within 20% tolerance. The method was applied to investigate chlorine isotope fractionation during alkaline hydrolysis of metolachlor, which showed a shift in δ 37 Cl of +46‰ after 98% degradation, demonstrating that chlorine isotope fractionation could be a sensitive indicator of transformation processes even when limited degradation occurs. This method, combined with large-volume solid-phase extraction (SPE), allowed application of Cl-CSIA to environmentally relevant concentrations of widespread herbicides (i.e. 0.5 to 5 µg/L in water before extraction). Therefore, the combination of large-volume SPE and Cl-CSIA is a promising tool for assessing the transformation processes of these pollutants in the environment. fractionation associated with reaction mechanisms relevant for field studies. Hence, future studies are warranted to better understand how variable dual isotope slopes are for specific processes in order to identify them unequivocally in the field. ASSOCIATED CONTENT Supporting Information Further details on target analytes, analytical and extraction methods, GC-qMS method performance and results of the validation tests are available in Supporting Information. The Supporting Information is available free of charge on the ACS Publications Website.