Abstract: Vacuum ultraviolet (VUV) photoionization mass spectrometry of antifungal drug terbinafine was investigated with IR laser desorption/VUV photoionization mass spectrometry (IR LD/VUV PIMS). By varying the VUV photon energy for ionization, the mass spectra of terbinafine at various photon energies were measured, and the formation pathways of dominant fragment ions were discussed.