A mass spectrometric method based on the combined use of electrospray ionization, collision-induced dissociation (CID) and ion-trap time-of-flight (IT-ToF) tandem mass spectrometry was used to identify four pairs of furocoumarin isomers. Our results show some characteristic mass spectrometric differences in isomeric furocoumarins. In the MS(2) spectra, the relative abundance ratios of m/z 131 [M + H - 2CO](+) to m/z 143 [M + H - CO(2)](+) and m/z 115 [M + H - CO-CO(2)](+) of psoralen are less than one, but both ratios are greater than one in the case of isopsoralen. The relative abundance of m/z 159 [M + H - CO(2)](+) is much higher than that of m/z 175 [M + H - CO](+) in the case of bergaptol, but this trend is reversed in xanthotoxol. Moreover, three diagnostic ions of m/z 189 [M + H - CO](+), m/z 185 [M + H - CH4O](+) and m/z 161 [M + H - 2CO](+) are observed in the MS(2) spectrum of xanthotoxin but not in that of bergapten. Further, two diagnostic ions of m/z 226 [M - H - CH(3) - CO](-) and m/z 210 [M - H - CH(3) - CO(2)](-) are observed in the MS(2) spectrum of isoimperatorin but not in that of imperatorin. The relative abundance of product ions associated with some diagnostic ions can be used to characterize furocoumarin isomers.