Chlorine isotope fractionation during preparative capillary gas chromatography (pcGC) was investigated using 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) as a model compound for semi-volatile organochlorine (OCl) molecules. Chlorine isotope analysis by thermal ionization mass spectrometry revealed no significant alteration of the chlorine isotope composition when the whole peaks were collected in pcGC (delta37Cl -3.2 per thousand versus -3.6 per thousand for the unprocessed DDT, +/-0.5 per thousand SD). However, distinct isotope fractionations were measured for the front (delta37Cl -5.1 per thousand) and tail (delta37Cl -1.8 per thousand) segments of partially collected samples. Isolation of individual OCls by pcGC enables accurate off-line chlorine isotope analysis, and thus facilitates the investigation of naturally occurring OCls.