Vacuum ultraviolet （VUV） photoionization and dissociative photoionization of capecitabine and its metabolites, 51-deoxy-5-fiuorocytidine （5＇-DFCR） and 51-deoxy-5- fiuorouridine （5＇- DFUR）, were investigated with infrared laser desorption/tunable synchrotron VUV photoionization mass spectrometry. Molecular ions （M＋） with small amounts of fragments can be found for these compounds at relatively low photon energies, while more fragment ions would be produced by increasing the photon energies. （M-H2O）＋, （base＋H）＋, （base＋2H）＋, （base＋30）＋, （base＋60）＋, and sugar moiety were proposed for these nucleoside drugs with similar backbones. Decomposition channels for the major fragments were discussed in detail. Moreover, ab initio calculations were introduced to study the dehydration pathways of three fluoro-nucleosides. Corresponding appearance energies for the （M-H2O）＋ ions were computed.
