Abstract: The antioxidant RD, widely used for rubber, has been determined and analyzed by GC-TOF-MS and TOF-MS using EI (electron ionization) source. It is found from TIC of GC-EIMS that the amount of dipolymer in RD is the largest with best thermal stability. Mass spectra has showed that it is easy to lose a methane molecule and a methyl group from each molecular ion in RD forming a conjugated ion; both di- and tripolymer often crack into a derivative ion of the dipolymer. The molecular ion peaks of all pleionomers in RD with different molecular weight can be appeared in the mass spectrum by heating gradually, from 100 ℃ to 600 ℃ of temperature during acquisition, introducing the solid sample RD with probe directly. Possible EI fragmentation pathways of RD and its monomer molecules have been presented and discussed. Therefore, this study forms the basis for the characterization of this kind of products and related classes of pleionomer molecules.