The microstructure of poly(vinylphosphonic acid) (PVPA) and poly(vinyl dimethyl phosphonate) (PVDMP) prepared by free radical polymerization was studied by a combination of one- and two-dimensional NMR spectra. Whereas C-13 and P-31 NMR spectra give less stereochemical information a signal split into diad, triad, and, at best, m-centered tetrad sequences for the methylene protons of PDMVP could be observed in the H-1 NMR spectra. For PVPA our assignments differ from those previously published in the literature. The spectra of both polymers were complicated by the presence of head-to-head regioirregular structures with up to similar to 17% for PVPA. For the regioregular part a mainly atactic structure was determined for PVPA, whereas PVDMP has an increased content of isotactic sequences. The dependency of H-1 and P-31 signals of PVPA on the degree of neutralization is presented.