Abstract The use of liquid chromatography—thermospray mass spectrometry (LC-TSP-MS) and liquid chromatography coupled with UV photodiode-array detection (LC-UV) in the analysis of crude plant extracts provides important structural information on metabolites directly in their biological matrices. In the LC analysis of polyphenols (such as xanthones), shift reagents can be added postcolumn and UV spectra recorded on-line. Information on the position of free hydroxyl groups can be obtained by adding reagents such as AlCl 3, weak and strong bases and boric acid. Thus, for certain polyphenols, the combination of LC-UV with postcolumn addition of shift reagents and LC-MS permits a full on-line structural determination involving no time-consuming isolation process. To illustrate this qualitative analytical approach, crude extracts of different Gentianaceae ( Chironia species) were submitted to LC-TSP-MS and LC-UV measurements with postcolumn addition of shift reagents. This method permitted the identification of a large number of xanthones which are of pharmaceutical interest as potential inhibitors of monoaminooxidases.