Abstract:The determination of negative and positive fast atom bombardment mass spectrometry (FAB-MS) and field desorption mass spectrometry (FD-MS) were carred out on 25 iridoidal glycosides.The result showed that negative FAB-MS is more useful for the determination of mo-lecular weight and the structural elucidation of iridoidal glycosides than positive FAB-MS and FD-MS.