Online monitoring of a miniemulsion polymerization of styrene by hyperpolarized Xe-129 NMR spectroscopy is presented. The chemical shift of Xe-129 directly reports on the monomer/polymer ratio in the reaction mixture and therefore on the conversion of the reaction. The method allows for monitoring the progress of a polymerization with high time resolution and without sample extraction. The results obtained by Xe-129 NMR spectroscopy were successfully validated by comparison with results of calorimetry. Further characterization of the polymer dispersions with regard to the solids content and the particle diameters proved the comparability of both methods. Small deviations in the kinetic data and the properties of the polymer suspension from both methods are explained by the loss of monomer during the Xe-129 NMR experiments due to the used setup. In the future, Xe-129 NMR might also be applied for online monitoring of other chemicals reactions, e.g., allowing for determining the kinetics of thermoneutral reactions.