Dimethyl vinyl phosphonate, diethyl vinyl phosphonate, and diisopropyl vinyl phosphonate were polymerized at 60°C via free radical polymerization to give oligomers. High-resolution electrospray ionization-mass spectrometry was used to analyze the resulting oligo(vinyl phosphonate)s in order to elucidate their polymerization mechanism. The resulting mass spectra showed no evidence of polymer chains capped with initiator derived fragments as end groups. Instead, the mass spectra indicated that transfer reactions generate activated monomer species (i.e., a monomelic radical), which then act as initiators in the polymerizations. Additionally, these studies indicate that termination takes place by chain transfer to monomer.