An HPLC method has been developed for the analysis of sesquiterpene lactones of the neurotoxic plant Centaurea solstitialis (Asteraceae). The presence of sesquiterpene lactone chlorohydrins in extracts was investigated by means of liquid chromatography-thermospray mass spectrometry. In contrast to earlier reports of a series of mono- and dichlorohydrins from this plant, traces only of two monochlorohydrins could be detected in lipophilic extracts. Model studies carried out with extracts and with the genuine sesquiterpene diepoxide repin and its epimer subluteolide showed that (i) monochlorohydrins can be formed in CHCl3 under usual laboratory conditions; (ii) the epoxide moiety at C-4 of the sesquiterpenes is extremely labile, reacting immediately and quantitatively with traces of HCl to the corresponding monohydrins; (iii) epoxide ring opening at the acyl side chain occurs only at higher HCl concentrations. Confirmation of the peak identity was obtained by the isotope ratio of chlorinated compounds and comparison with authentic samples. The structures of the mono- and dichlorohydrins were established by NMR spectroscopy.