In-line monitoring of monomer conversion is studied using a low-frequency dielectric sensor during solution polymerization of methyl methacrylate in a laboratory-scale batch reactor. It has been found that the induction period due to the presence of an inhibitor can be determined by the dielectric sensor to detect the starting point of polymerization. The dielectric loss factor that changes with the progress of polymerization is empirically correlated with monomer conversion measured by the off-line gravimetric method at different temperatures and initiator concentrations. It is demonstrated that dielectrometry can be used as an in-line monitoring method in free-radical polymerization.