Polymerization of n-hexyl methacrylate (C6MA) in aqueous microemulsions made with mixed dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) surfactants yields very small (∼30 nm) latex particles of high molecular weight polymer. The results of an online small-angle neutron scattering (SANS) experiment have verified some of the assumptions of a simple but accurate kinetic model that has been recently proposed for aqueous microemulsion polymerization. The SANS results also support the predictions of an analytical model that describes the evolution of the chain length and particle size distribution throughout the polymerization. During the microemulsion polymerization of C6MA, the monomer does not significantly swell the polymer particles. A model of the reacting particle as a polymer core surrounded by a monomer-rich shell is consistent with both the kinetic and SANS data if the monomer concentration in the shell is equal to that in the core of the swollen micelles.