To achieve a better understanding of the kinetics of microemulsion polymerization, we have examined monomer partitioning between micelles and polymer particles during polymerization of aqueous microemulsions of dodecyltrimethylammonium bromide surfactant and styrene, n-butyl methacrylate, tert-butyl methacrylate: and n-hexyl methacrylate monomers. Polymerizing microemulsions are simulated by equilibrium mixtures of unpolymerized and fully polymerized microemulsions for which the monomer partitioning is accurately measured using small-angle neutron scattering. Experimental results are semiquantitatively reproduced by modeling the monomer partitioning as a competition between the Flory-Huggins bulk polymer free energy and the Helfrich curvature elastic energy of the surfactant monolayers.