Electron beam‐induced polymerization of trimethylolpropane triacrylate (TMPTA) and its methacrylate analog (TMPTMA) was studied using nuclear magnetic resonance (NMR) relaxation time measurements. Free induction decays (FID) of partially polymerized samples consist of a short Gaussian component and a longer component comprised of a distribution of simple exponentials. The relative intensity of the Gaussian component increases with radiation dose. T1 and T1ρ values were measured as a function of temperature and radiation dose. The relaxation is due primarily to methyl group reorientation at low temperatures, ethyl group reorientation at intermediate temperatures, and whole‐molecule reorientation at high temperatures. In both compounds, the T1 and T1ρ values at the high temperature minima increase with increasing dose, and the minima values can be used to estimate the degree of polymerization. The temperature at which the T1ρ minimum occurs increases with dose, suggesting an increase in the glass transition temperature, Tg, with polymerization. The polymerization appears to have very little effect on the low temperature CH3 reorientation in TMPTA. In TMPTMA the polymerization appears to reduce the mobility of the methacrylate methyl groups.